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
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### Further details
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### Proposal
Standardize on a syntax, and render these as disabled checkboxes.
Disabled tasks should not be included in the count of "tasks completed".
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### Permissions and Security
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### Documentation
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The chosen syntax should be documented at https://docs.gitlab.com/ee/user/markdown.html#task-lists.
### Testing
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### What does success look like, and how can we measure that?
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### Links / references
issue