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/toast-and-message.tmpl

@@ -0,0 +1,11 @@
+{{template "devtest/devtest-header"}}
+<div class="ui container">
+	<h1>Toast</h1>
+	<div>
+		<button class="ui button toast-test-button" data-toast-level="info" data-toast-message="test info">Show Info Toast</button>
+		<button class="ui button toast-test-button" data-toast-level="warning" data-toast-message="test warning">Show Warning Toast</button>
+		<button class="ui button toast-test-button" data-toast-level="error" data-toast-message="test error">Show Error Toast</button>
+		<button class="ui button toast-test-button" data-toast-level="error" data-toast-message="very looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong message">Show Error Toast (long)</button>
+	</div>
+</div>
+{{template "devtest/devtest-footer"}}