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/user_settings_test.go

@@ -5,6 +5,7 @@ package integration
 
 import (
 	"net/http"
+	"strings"
 	"testing"
 
 	"code.gitea.io/gitea/modules/container"
@@ -309,8 +310,7 @@ func TestUserSettingsApplications(t *testing.T) {
 				})
 				resp := session.MakeRequest(t, req, http.StatusOK)
 				doc := NewHTMLParser(t, resp.Body)
-
-				msg := doc.Find(".flash-error p").Text()
+				msg := strings.TrimSpace(doc.Find(".ui.message.flash-message").Text())
 				assert.Equal(t, `form.RedirectURIs"ftp://127.0.0.1" is not a valid URL.`, msg)
 			})