Clean up various avatar dimensions (#24701)

Clean up a few cases where avatar dimensions were overwritten via CSS,
which were no longer needed or were possible to set via HTML width.

Also included are two small fixes:

- Fix one more case of incorrect avatar offset on review timeline
- Vertically center avatars in review sidebar

There is more to be done here, but some of the work depends on Fomantic
`comment` module removal, or in the case of org member lists, a refactor
of the `avatarlink` template to accept a size.

<img width="371" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/9c5902fb-2b89-4a7d-a152-60e74c3b2c56">
<img width="306" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/c8d92e2a-91c9-4f4a-a7de-6ae1a6bc0479">

---------

Co-authored-by: Giteabot <teabot@gitea.io>

templates/shared/issuelist.tmpl

@@ -153,7 +153,7 @@
 				<div class="issue-item-icon-right text grey">
 					{{range .Assignees}}
 						<a class="ui assignee gt-tdn" href="{{.HomeLink}}" data-tooltip-content="{{.GetDisplayName}}">
-							{{avatar $.Context .}}
+							{{avatar $.Context . 20}}
 						</a>
 					{{end}}
 				</div>