Refactor flash message and remove SanitizeHTML template func (#37179)

1. Fix the "flash message" layout problem for different cases
* I am sure most of the users should have ever seen the ugly
center-aligned error message with multiple lines.
2. Fix inconsistent "Details" flash message EOL handling, sometimes
`\n`, sometimes `<br>`
   * Now, always use "\n" and use `<pre>` to render
3. Remove SanitizeHTML template func because it is not useful and can be
easily abused.
* But it is still kept for mail templates, for example:
https://github.com/go-gitea/gitea/issues/36049
4. Clarify PostProcessCommitMessage's behavior and add FIXME comment

By the way: cleaned up some devtest pages, move embedded style block to
CSS file

templates/repo/commit_page.tmpl

@@ -195,7 +195,7 @@
 				<span class="tw-text-text-light">{{DateUtils.TimeSince .NoteCommit.Author.When}}</span>
 			</div>
 			<div class="ui bottom attached info segment git-notes">
-				<pre class="commit-body">{{.NoteRendered | SanitizeHTML}}</pre>
+				<pre class="commit-body">{{.NoteRendered}}</pre>
 			</div>
 		{{end}}