Review the layout to always use standard bootstrap classes so that
future task targeting modal can target s_popup snippets the same way
(for example: our task allowing to open a popup on click on a button).

This also allows to simplify the css and simplify the options. A new
option also now allows to have a backdrop, or not, so that user clicks
are blocked while the popup is opened, or not.

+ Adapt cookie bar accordingly.

Part of https://github.com/odoo/odoo/pull/52698
task-2210730

Co-authored-by: qsm-odoo <qsm@odoo.com>

With this commit, it is now possible to activate a cookies bar on a
website through the res.settings.

The cookies bar can be edited in the frontend by entering edit mode.

The cookies bar will appear until the user clicks on 'I agree'.

Part of https://github.com/odoo/odoo/pull/41294
task-2087003

closes odoo/odoo#41294

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>