[REF] partial backport of 777a082

[IMP] website: create customize theme application and color-picker
This is quite a huge commit and as we don't want to break the
backward compatibility with css bootswatch, we did not import
the theme_bootswatch module
[REF] partial backport of 2d296cb7

only on website/views/themes.xml

[FIX] reintroduce website.theme included in bundle
[REF] backport of 7210200de0b97ad

[FIX] Fix website theme slowness
[REF] partial backport of c2677880400b988
[FIX] website: add a bool function field `option_bootstrap_less`

Used in website themes.
This field will try to browse the `option_bootstrap_less` view and
read its active field. It allows to hide/show the `customize_theme`
option in customize without adding a group or a field.

Also, added a method in website res_config to switch between legacy
bootswatch css and less themes.