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.
Add specs for tasks

Add tests for the new task list functionality in the Markdown parser and
in issues and merge requests.
Update docs for task lists
Add Spinach tests for task lists
Merge pull request #7964 from mr-vinn/task-lists

Add task lists to issues and merge requests