[WIP] Add task lists to issues and merge requests

Make the Markdown parser recognize "[x]" or "[ ]" at the beginning of a list item and turn it into a checkbox input.  Users who can modify the issue or MR can toggle the checkboxes directly or edit the Markdown to manage the tasks.  Task status is also displayed in the MR and issue lists.  I still need to write tests and update the documentation, but the functionality is complete.
Unused block argument - `match`. You can omit the argument if you don't care about it.