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

modules/templates/helper_test.go

@@ -58,7 +58,7 @@ func TestSubjectBodySeparator(t *testing.T) {
 }
 
 func TestSanitizeHTML(t *testing.T) {
-	assert.Equal(t, template.HTML(`<a href="/" rel="nofollow">link</a> xss <div>inline</div>`), SanitizeHTML(`<a href="/">link</a> <a href="javascript:">xss</a> <div style="dangerous">inline</div>`))
+	assert.Equal(t, template.HTML(`<a href="/" rel="nofollow">link</a> xss <div>inline</div>`), sanitizeHTML(`<a href="/">link</a> <a href="javascript:">xss</a> <div style="dangerous">inline</div>`))
 }
 
 func TestTemplateIif(t *testing.T) {