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/issue/view_content/comments.tmpl

@@ -164,7 +164,7 @@
 				<div class="detail flex-text-block">
 					{{svg "octicon-git-commit"}}
 					{{/* the content is a link like <a href="{RepoLink}/commit/{CommitID}">message title</a> (from CreateRefComment) */}}
-					<span class="comment-text-line">{{.Content | SanitizeHTML}}</span>
+					<span class="comment-text-line">{{.GetSanitizedContentHTML}}</span>
 				</div>
 			</div>
 		{{else if eq .Type 7}}