Button and color enhancements (#24989) (#25176)

Backport #24989. Clean cherry-pick aside from one small conflict with
divider.

- Various corrections to button styles, especially secondary
- Remove focus highlight, it's annoying when it stays on button after
press
- Clearly define ghost and link buttons with demos in devtest
- Remove black, grey and tertiary buttons, they should not be used
- Make `arc-green` slightly darker

<img width="1226" alt="image"

src="8d89786a-01ab-40f8-ae5a-e17f40e35084">
<img width="1249" alt="image"

src="83651e6d-3c27-46ff-b8bd-ff344d70e949">

templates/repo/commit_statuses.tmpl

@@ -1,6 +1,6 @@
 {{if .Statuses}}
 	{{if and (eq (len .Statuses) 1) .Status.TargetURL}}
-		<a class="gt-vm gt-tdn" data-tippy="commit-statuses" href="{{.Status.TargetURL}}">
+		<a class="gt-vm gt-no-underline" data-tippy="commit-statuses" href="{{.Status.TargetURL}}">
 			{{template "repo/commit_status" .Status}}
 		</a>
 	{{else}}