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/repo.css

@@ -1823,16 +1823,12 @@
   clear: right;
 }
 
-.repository .diff-file-box.file-content img {
+.repository .diff-file-box.file-content .image-diff img {
   max-width: 100%;
   padding: 0;
   border-radius: 0;
 }
 
-.repository .diff-file-box.file-content img.emoji {
-  padding: 0;
-}
-
 .repository .diff-file-box .ui.bottom.attached.table.segment {
   padding-top: 5px;
   padding-bottom: 5px;
@@ -3085,8 +3081,7 @@ tbody.commit-list {
 }
 
 .diff-file-header {
-  padding-top: 5px !important;
-  padding-bottom: 5px !important;
+  padding: 5px 8px !important;
 }
 
 .diff-file-box[data-folded="true"] .diff-file-body {