[REF] partial backport of 777a082bfc8d

[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] 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
[FIX] restore change_theme
[REF] Partial backport of c2677880400b988c5dc1b779edb2d7736e81aafe
[FIX] remove application from backport of al's commit previously
[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.