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

templates/shared/issuelist.tmpl

@@ -84,7 +84,7 @@
 					{{end}}
 					{{if ne .DeadlineUnix 0}}
 						<span class="due-date" data-tooltip-content="{{$.locale.Tr "repo.issues.due_date"}}">
-							<span{{if .IsOverdue}} class="overdue"{{end}}>
+							<span{{if .IsOverdue}} class="text red"{{end}}>
 								{{svg "octicon-calendar" 14 "gt-mr-2"}}
 								{{DateTime "short" .DeadlineUnix}}
 							</span>