Rework repo buttons (#20602)

* Rework repo buttons

- Replace "New PR" and "Go to File" button with Icon Button
- Move all "Add File" actions into a dropdown button
- Remove most custom styling of clone buttons
- Margin and wiki tweaks

Buttons are now all equal height, mobile layout wraps gracefully.

Fixes: https://github.com/go-gitea/gitea/issues/13671
Replaces: https://github.com/go-gitea/gitea/pull/20375

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/helpers.less

@@ -17,6 +17,7 @@
 .vm { vertical-align: middle !important; }
 .w-100 { width: 100% !important; }
 .h-100 { height: 100% !important; }
+.br-0 { border-radius: 0 !important; }
 
 .mono {
   font-family: var(--fonts-monospace) !important;