Set opaque background on markup and images (#23578)
- Set opaque background on markup images so they can visually break
`<hr>`
- Change padding of comment box so `padding` is provided by the
`.markup` element instead of its parent, matching the file rendering
view which does the same.
Before:
<img width="243" alt="Screenshot 2023-03-19 at 19 22 03"
src="https://user-images.githubusercontent.com/115237/226198663-8ff4d940-6a15-452d-ac58-14485b37fbc7.png">
After:
<img width="261" alt="Screenshot 2023-03-19 at 19 23 26"
src="https://user-images.githubusercontent.com/115237/226198689-1bf56561-4726-46dc-b583-423d65e1e13a.png">
<img width="263" alt="image"
src="https://user-images.githubusercontent.com/115237/226199002-e93c817d-6d9c-4b98-bad8-0aa0bd45b62f.png">
Example documents:
https://try.gitea.io/silverwind/symlink-test/src/branch/master/test-page.md
https://github.com/silverwind/symlink-test/blob/master/test-page.md
ca0ce9feb0
