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/devtest/devtest-header.tmpl

@@ -1,4 +1,4 @@
 {{template "base/head" ctx.RootData}}
 <link rel="stylesheet" href="{{AssetURI "css/devtest.css"}}">
 <div class="tw-hidden" data-global-init="initDevtestPage"></div>
-{{template "base/alert" .}}
+<div class="ui container tw-mt-4">{{template "base/alert" ctx.RootData}}</div>