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

@ulferts yes. Better theming is beyond the scope of this PR.