Description

As part of this PR Bootstrap is upgraded from version 3 to version 4. Also we are simplifying it's integration into the core.

Particularly we completely remove the less/sass meta layer for these main reasons:

1. Much more complexity than needed: basically we use Bootstrap, but we don't extend or amend it significantly.
   • We want to stay close to the standard.
   • The scss files are really many and we just don't want to fiddle around with them.
2. More update pain: the component repo does not include any sass/scss sources atm (for similar reasons).
3. Bootstrap 4 does not contain glyph icons anymore (which we usually removed in favour of font awesome). Instead they recommend using FA now, too.

So from now on we just treat Bootstrap as what it is - a vendor component - and work with it instead on it.

If somebody wants to further customize things he can still do it. We provide the possibility to use a custom bootstrap css path for that. But Zikula does not have to provide the "source" experience of the whole BS package ootb.

This PR also removes the "zikula.stylesheet.bootstrap-font-awesome.path" because there is no need to have this in one file then anymore. Asset combination can do this instead if desired.

Todos

• Check that latest BS release is available in component repo (created PR: Updated to version 4.4.1 components/bootstrap#59)
• Completely remove the less/sass meta layer
• Remove the "zikula.stylesheet.bootstrap-font-awesome.path"
• We have some simple cosmetic overrides in our bootstrap_variables. Review them and either remove them or possibly put them into core.css for the moment.
• Check if modal fix for mmenu is still needed.
• Adapt FormExtensionBundle to Symfony's Bootstrap 4 Form Theme
• Upgrade bundle and module templates
• Remove Andreas08 and SeaBreeze themes
• Upgrade Bootstrap theme
• Extend Bootstrap theme to support not only cerulean multiple themes from https://bootswatch.com/ - at the end these are only different css files and they support composer
• Provide an additional Bootstrap theme variable (checkbox option) to allow users switching between Bootswatch styles using a dropdown. Unsure whether it's worth the effort (maybe something for later).