[IMP] website, *: optimize scss customizations

* web_editor

This commit's goal is to optimize the delay to make a scss customization
thanks to the website customize dialog. Before this commit, the JS code
took advantage of existing web_editor routes but this was far from
efficient as this was done:

1) PY: Load all scss files used in the page
2) JS: Find the one we want to customize and adapt the scss content
3) PY: Create the scss customization with the new content

(So two RPC (including one very slow) and content building in
javascript)

After this commit, this is done:

1) PY: Load the scss file being customized, adapt its content according
   to the new values and save the customization

(So the whole logic is in python, requiring only one small RPC)

This commit also take the opportunity to review the web_editor ace
editor loading code (the logic is shared with website customizations).

PR https://github.com/odoo/odoo/pull/29624
task-1904244