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

web_src/css/markup/content.css

@@ -3,6 +3,7 @@
   font-size: 16px;
   line-height: 1.5 !important;
   word-wrap: break-word;
+  background: var(--color-box-body);
 }
 
 .markup.ui.segment {
@@ -308,6 +309,7 @@
 .markup img {
   max-width: 100%;
   box-sizing: initial;
+  background: var(--color-box-body);
 }
 
 .markup img[align="right"] {