Render plaintext task list items for markdown files (#26186)

- The library that's being used for org-mode, [doesn't render the status
of list items](https://github.com/niklasfasching/go-org/issues/63).
- Add a modified version of the proposed CSS snippet to still display
the status for the list items. The alternative was parsing HTML and
transforming it, which is too complicated for this small task.
- Resolves https://codeberg.org/Codeberg/Community/issues/1099

(cherry picked from commit 9753c7e4b8)

Refs: https://codeberg.org/forgejo/forgejo/pulls/1071

Co-authored-by: Gusted <postmaster@gusted.xyz>

modules/markup/sanitizer.go

@@ -96,6 +96,9 @@ func createDefaultPolicy() *bluemonday.Policy {
 	// Allow classes for task lists
 	policy.AllowAttrs("class").Matching(regexp.MustCompile(`task-list-item`)).OnElements("li")
 
+	// Allow classes for org mode list item status.
+	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^(unchecked|checked|indeterminate)$`)).OnElements("li")
+
 	// Allow icons
 	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^icon(\s+[\p{L}\p{N}_-]+)+$`)).OnElements("i")