Support for subtasks

[willbarkoff]

It would be amazing if MHS supported subtasks for assignments. Right now, I need to do an evaluation for everyone on a team I'm on, so overview currently looks like this

I've blacked out each name, but because I have to do an eval for each person, and I want to keep track of who i've done so far, they're each their own assignment. Ideally, I'd have one big assignment that encompasses all of them, with checkboxes inside.

It would be cool if we could implement this along with #180, in a similar manner to GitHub issues. You can create a checklist in the body of a GitHub issue, and it tracks the progress for you.

[thatoddmailbox]

This is sort of related to #170 (or at least my response to that issue). I think the main problem would be figuring out how to do something logical in dashboard/planner when the different parts are due on different days? Or even if they were due on the same day I'm not really sure how it would appear in either of those views