[REF] web, *: review the assets helpers organization

* survey, website_slides

While not changing their name or their amount, this commit reorganizes
the assets helpers to:

Avoid all file duplicates in different helpers
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Before, we had to put some files in different helpers, like our
bootstrap mixin overrides which were in the _assets_utils template and
in the _assets_bootstrap template.

Enforce the use of an helper when using bootstrap
-------------------------------------------------

Bootstrap functions, variables and mixins are now out of the
_assets_bootstrap template, which now only contains bootstrap css rules
and our bootstrap fixes and extensions.

Allow using the correct bootstrap utils in all assets
-----------------------------------------------------

In some assets (especially editor ones), we may want to use bootstrap
variables even though bootstrap is not included in those. This worked
before but using *default* bootstrap variables. Now, our overrides can
be properly included if necessary.

Note: this commit may also be a first step to allow for backend
bootstrap and frontend bootstrap to co-exist.

closes odoo/odoo#28204

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