Project board improvements (#15429)

Project board improvements (#15429)

* Project board improvements

- Fix link colors
- Extract CSS to own file
- Various minor tweaks to make it look better

Fixes: https://github.com/go-gitea/gitea/issues/15424
Fixes: https://github.com/go-gitea/gitea/issues/15506
Fixes: https://github.com/go-gitea/gitea/pull/15511
* fix squashed cards on small view area

* more css fixes, add second row from issue list

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>