Dropdowns, Labels fixes and more CSS tweaks (#13733)

Dropdowns, Labels fixes and more CSS tweaks (#13733)

* Dropdowns and Labels fixes

- Rework dropdown, menu and label styles
- Improve issue sidebar milestone and label sections
- Fix archived repo and private org badge
- Move more colors to CSS vars
- Move issue number to end of title on issue page

* more dropdown fixes

* fix basic blue labels - fixes #13731

* improve class setting on svg

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>