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/branch.go

@@ -231,7 +231,7 @@ func CreateBranch(ctx *context.Context) {
 				flashError, err := ctx.RenderToHTML(tplAlertDetails, map[string]any{
 					"Message": ctx.Tr("repo.editor.push_rejected"),
 					"Summary": ctx.Tr("repo.editor.push_rejected_summary"),
-					"Details": utils.SanitizeFlashErrorString(e.Message),
+					"Details": utils.EscapeFlashErrorString(e.Message),
 				})
 				if err != nil {
 					ctx.ServerError("UpdatePullRequest.HTMLString", err)