Search and Diff CSS enhancements (#14050)

Search and Diff CSS enhancements (#14050)

* Search and Diff CSS enhancements

- Use flexbox for language stats
- Improve labels and code boxes on repo and code search
- Use flexbox on diff header and improve suppressed diff text
- Add dedicated color for diff expander

* more diff tweaks, less vertical padding on header

* more minor tweaks

* always show fold icon, image diff improvments

* remove margin

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