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">

web_src/css/shared/issuelist.css

@@ -145,7 +145,7 @@
 }
 
 .issue.list .branches .branch {
-  background-color: var(--color-secondary);
+  background-color: var(--color-secondary-alpha-40);
   border-radius: 3px;
 }