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

routers/web/repo/commit.go

@@ -410,7 +410,8 @@ func Diff(ctx *context.Context) {
 		ctx.Data["NoteCommit"] = note.Commit
 		ctx.Data["NoteAuthor"] = user_model.ValidateCommitWithEmail(ctx, note.Commit)
 		rctx := renderhelper.NewRenderContextRepoComment(ctx, ctx.Repo.Repository, renderhelper.RepoCommentOptions{CurrentRefPath: path.Join("commit", util.PathEscapeSegments(commitID))})
-		ctx.Data["NoteRendered"], err = markup.PostProcessCommitMessage(rctx, template.HTMLEscapeString(string(charset.ToUTF8WithFallback(note.Message, charset.ConvertOpts{}))))
+		htmlMessage := template.HTML(template.HTMLEscapeString(string(charset.ToUTF8WithFallback(note.Message, charset.ConvertOpts{}))))
+		ctx.Data["NoteRendered"], err = markup.PostProcessCommitMessage(rctx, htmlMessage)
 		if err != nil {
 			ctx.ServerError("PostProcessCommitMessage", err)
 			return