Various UI fixes (#25264)

Numerous small UI fixes:

- Fix double border in collaborator list
- Fix system notice table background
- Mute links in repo and org lists
- Downsize projects edit buttons
- Improve milestones and project list rendering
- Condense milestone list entry to a single line of "metas"
- Mute ".." button in repo files list

web_src/css/shared/issuelist.css

@@ -135,10 +135,6 @@
   margin-right: -4px;
 }
 
-.issue.list > .item .desc .overdue {
-  color: var(--color-red);
-}
-
 .issue.list .branches {
   display: inline-flex;
   padding: 0 4px;