This PR is a first step towards a potential deletion of the
assets_common bundle, although that step would need more work, specs and
discussions as some layouts kinda only use the assets_common bundle
(some take the full assets_common but parts of the assets_backend one
for example).

The main goal of this commit is to have the assets_frontend bundle
directly include the "common" files we need. As a first step, this
commit only blindly duplicates them all into assets_frontend (without
removing the potentially useless ones). The goal is to have those
advantages:

- Reaching a frontend page only calls two main JS files (one normal and
  one lazy-loaded) instead of 4 (two normals and two lazy-loaded). This
  may help reach a better google page speed (which is becoming more and
  more strict).

- The frontend CSS is built as one: the common SCSS which was using
  bootstrap variables, or even Odoo-based SCSS added by mistake in
  common instead of both backend and frontend is now computed with the
  right bootstrap customizations. E.g. the tempusdominus datetimepickers
  use bootstrap grays... after this PR, they use the right grays as
  customized by the user on the website.

It was also chosen to not have a common "sub-asset" which is included in
assets_frontend. Making assets_frontend completely independent makes
sense (as it probably will for other "main" asset bundles): we can focus
on adding the files each layout needs without the need of worrying if it
impacts unrelated layouts. Sub-assets (when not strictly necessary) is
also a source of errors: extending the "main" bundle instead of the
right sub-asset it may use (like it was the case with the sub-assets of
assets_common: _assets_common_scripts and _assets_common_styles as
explained in the previous commit). So this is indeed a small drawback of
not factorizing the code for the inclusion of "common" files in bundles
but it seems more explicit and easier to maintain that way. Note that
adding "common" file is not the most common usecase anyway, apps
generally only need files in backend or frontend.
The __manifest__ declaration will also likely evolve in more and more
uses of wildcards to match entire directories. In the future, adding
"common" web-app files in both backend, frontend and other "main"
bundles could just be about one line duplicated into each bundle.