* theme_bootswatch, theme_default, website_theme_install

Follow up of commit a3845ae3f058772d4b548c703f5ddf94b571d903

Issue
=====

Since the above commit there is a confirmation dialog when clicking on a button,
for which the only options are to Discard or Cancel.

The problem is that when discarding it will revert the selected website back to
the default value. Therefore before the current commit it is impossible to
install a theme on a website other than the first.

Solution
========

The solution is to avoid showing this dialog when clicking on buttons for which
the result depends on the currently selected website. This leads to the same
behavior as what existed before the mentioned commit: the settings are
implicitly discarded because they are not saved, but at the time of the click
the transient model still holds the non-discarded value that the user selected.

This restores the "select a theme" feature, but also restores the unfortunate
side effect that the user might unknowingly loose all of his pending changes
when clicking on the button, but only for those two specific buttons.

Other considerations
====================

Saving instead of Discarding was considered but it is impossible to do because
saving might do a lot of changes that might even lead to the installation of
modules, so the settings page has to be reloaded to apply them, and then the
website field will be reset to its default value again.

Instead of changing website specific settings from the main settings, it was
considered to update them through a form view, but it is apparently too
complicated for on-boarding.

The last consideration was that the settings view should actually mark the
corresponding fields as unchanged when changing the website, but it is too
complex to implement for a case that is not supported in the first place,
especially for a fix in a stable version.

task-2083198

closes odoo/odoo#38745

X-original-commit: 2298ef19bcd9e7a14958dce9a93cea69258064d8
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

task-1907934

closes odoo/odoo#31114

From now, when you install a theme, it load the data from xml to template
table theme [ir.ui.view|ir.attachment|website.page|website.menu].

Data are only copied from this template table into the real table when you
choose a theme on a website; Making them website_specific and with a link
to the original to allow futur update.

A special case is done to create theme.ir.ui.view when you are installing
a theme, even if you continue to use template tag to create quick view.

Co-authored-by: Derie Romain <rde@odoo.com>
Co-authored-by: Kersten Jérémy <jke@odoo.com>

- Uniform colors and design
- Replace duplicated icons (eg. sale / sale_management)
- Improve misleading icons (eg. POS)
- Add icons for new apps

Add SVG versions to lossless future editing and print/marketing use.

task-54681

Odoo used to declare two main colors: primary and optional (which are
purple and turquoise in enterprise). Those were respectively assigned
to the 'primary' bootstrap variable and the 'btn-primary' bootstrap
variable.

BS4, however, does not allow to have a different primary color for
buttons. Instead, the 'primary' color is used for all 'primary' related
components and utility classes, same as for all other colors. So, if we
want to keep our enterprise buttons green, our 'optional' colors had to
become our 'primary' color. The old odoo primary is then renamed to the
'odoo' color.

The palette of grays is now larger by default and is numbered from 100 to
900 alongside the $black and $white variables. The equivalence for older
variables and the way we used them is:

$gray-darker          ->  gray 900 (unused before)
$gray-dark            ->  gray 900
$gray                 ->  gray 700
$gray-light           ->  gray 600
$gray-lighter-darker  ->  gray 400 (the old variable was created by us)
$gray-lighter-dark    ->  gray 300 (the old variable was created by us)
$gray-lighter         ->  gray 200

Fortunately, the 'lighter' variations we created fit well in the default
BS4 system ! Unfortunately, our $gray-lighter which carried the same
function as $gray-200 (see above) is very close to the new default
$gray-100 and quite distant from the new $gray-200. This will be handled
in the next commit.

BS variables were renamed or changed and all (most) variables now
use 'rem' units instead of 'px' unit.

- The dropdown structure was simplified, allowing to get rid of the
  3-levels structure induced by <ul/> elements and dropdowns can now
  contain anything. The class 'dropdown-item' is now mandatory for
  each dropdown clickable element. The class 'dropdown-item-text' can
  be used to add same padding and style but without making the element
  have a clickable look.

- Dividers now use the class 'dropdown-divider'

- The way dropdowns are opened and hidden also changed (before the
  'open' class was added on the `.dropdown-menu` parent, now the
  'show' class is added on both the `.dropdown-menu` parent and the
  `.dropdown-menu` itself).

- JS-wise, no click event handlers can be put on `.dropdown-toggle`
  elements anymore (instead, use handlers for dropdown events).

- Carets are automatically put on `.dropdown-toggle` elements, so this
  commit replaces the `.caret` elements with this. This feature was
  possible to disable but would prevent us from adding a caret with
  scss. Also, this simplifies the DOM. The 'o-no-caret' class was also
  introduced to allow using the 'dropdown-toggle' class on non-caret
  elements.

- Also adapt the scss to use $caret-width instead of $caret-width-base