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

tests/integration/pull_merge_test.go

@@ -317,9 +317,8 @@ func TestPullCleanUpAfterMerge(t *testing.T) {
 		resp = session.MakeRequest(t, req, http.StatusOK)
 
 		htmlDoc := NewHTMLParser(t, resp.Body)
-		resultMsg := htmlDoc.doc.Find(".ui.message>p").Text()
-
-		assert.Equal(t, "Branch \"user1/repo1:feature/test\" has been deleted.", resultMsg)
+		resultMsg := strings.TrimSpace(htmlDoc.doc.Find(".ui.message.flash-message").Text())
+		assert.Equal(t, `Branch "user1/repo1:feature/test" has been deleted.`, resultMsg)
 	})
 }