Backport #25292 by @silverwind
Save another 50KB of CSS by removing unused and useless Fomantic
variants.
Removed the last instance of a `tertiary` button and fixed a TODO:
<img width="509" alt="Screenshot 2023-06-15 at 22 34 36"
src="8a16ae7b-2b17-439b-a096-60a52724e3d6">
Co-authored-by: silverwind <me@silverwind.io>
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">
1. Add this button:
<img width="232" alt="Screenshot 2023-05-29 at 15 21 47"
src="5eaf6bd1-83db-4ffc-9503-eda0c59807d2">
<img width="297" alt="Screenshot 2023-05-29 at 15 20 22"
src="708a344f-f6d7-4229-bfda-76e1571b42c8">
2. Correct `button-link` styles to not have a background hover effect.
3. Tweak `.ui.container` padding to be the same for fluid and non-fluid.
4. Misc enhancements to diff header:
Before:
<img width="984" alt="Screenshot 2023-05-29 at 15 38 53"
src="c7926f6a-bd0a-4b05-97ad-c91fc25c62d5">
After:
<img width="987" alt="Screenshot 2023-05-29 at 15 43 10"
src="0149f545-45f8-42cf-b443-e1c76bd5cdeb">
