[IMP] website: create customize theme application and color-picker

This is a partial backport of 777a082bfc8d
[REF] backport of 2d296cb7 only on website/views/themes.xml
[FIX] reintroduce website.theme included in bundle
[FIX] Fix website theme slowness
[FIX] restore change_theme
[REF] Partial backport of c2677880400b988c5dc1b779edb2d7736e81aafe
[FIX] remove application from backport of al's commit previously
FIXME DMO