The refactoring of the wysiwyg editor and 'html' field allows us to
move some of the code that was in web_editor but only used in website or
in mass_mailing. Some parts are still in web_editor but will be
moved at a later time.

Purpose behind this commit is to remove font-family from all elements in
plaintext theme. So mail receiver will see this mail in default system font.
This is inspired by new Gmail and suggested by FP

As we want to show system's default font in the mail sent with the basic theme.
And so we remove font-family from basic theme while saving. But by doing so in
readonly mode is not getting proper font in odoo as we don't having any assets in
readonly mail editor. So we added stylesheet just for display purpose.

- 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

The system completely changed. I also had to adapt classes to new
screen breakpoints.

hidden/hide -> d-none
show -> d-block
hidden-xs -> d-none d-md-(block/inline/...)
hidden-sm -> d-md-none d-lg-(block/inline/...)
hidden-md -> d-lg-none d-xl-(block/inline/...)
hidden-lg -> d-xl-none
visible-xs-* -> d-* d-md-none
visible-sm-* -> d-none d-md-* d-lg-none
visible-md-* -> d-none d-lg-* d-xl-none
visible-lg-* -> d-none d-xl-*
hidden-print -> d-print-none
visible-print-* -> d-none d-print-*
...
and all possible combination of those had to be handled too.

* web_editor, website

This commit fixes a bug with table duplication in mailing template:
- Click on a snippet with two columns
- Duplicate it
-> the snippet is duplicated but the two columns are transformed in two
   rows.

This was because there is an implemented behavior for column
duplication: if we duplicate a column which was alone on its row, we
duplicate the row instead. This code should however be triggered when
duplicating the column alone to be working, not the snippet that
contains it.

Note: this required a backport of a33ec972abf1cadbae2a6a04f759617c82f2a95e
which was fixing a non-critical "bug" for 11.0. This non-critical "bug" is
therefor fixed here too.

opw-807105

- There should not be margins around lists
- There should not be editor overlay on the whole content