Integrate Foundation for Apps framework

https://community.openproject.org/work_packages/17751

* [x] Layout fixes (major issues)
* [x] News fixes
* [x] Buttons
* [x] Update OP plugins/themes

Moved to #2359 / other PRs:
* Filter design improvements
* Button groups, segmented buttons (in toolbar)
* Typography (normalisation) 
* Progress bars

How does this relate to https://github.com/opf/openproject/pull/2353? Will the menu on wiki et. al. when using themes be lost again.