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/repo/projects/view.tmpl

@@ -49,7 +49,7 @@
 			</div>
 			{{if and $.CanWriteProjects (not $.Repository.IsArchived)}}
 				<div class="column right aligned">
-					<div class="ui compact right small menu">
+					<div class="ui compact right mini menu">
 						<a class="item" href="{{$.RepoLink}}/projects/{{.Project.ID}}/edit?redirect=project">
 							{{svg "octicon-pencil"}}
 							<span class="gt-mx-3">{{$.locale.Tr "repo.issues.label_edit"}}</span>