Support disabled tasks in issues/MRs

Problem to solve

Sometimes a template contains tasks which are not relevant to the issue/MR. Rather than just checking them off or deleting them, I think it's better to indicate that they're not necessary. I personally adapted the - [-] syntax for this which I've seen others do.

Unfortunately these are rendered as-is and look a bit out of place:

Intended users

Further details

Proposal

Standardize on a syntax, and render these as disabled checkboxes.

Disabled tasks should not be included in the count of "tasks completed".

Permissions and Security

Documentation

The chosen syntax should be documented at https://docs.gitlab.com/ee/user/markdown.html#task-lists.

Testing

What does success look like, and how can we measure that?

Links / references