This PR adds a marketplace for file actions.

How it looks on mobile

Changelog

• Add a description field to FileAction
  • Right now it's a CharField(max_length=100), it should be discussed whether a TextField or a CharField with a larger length limit is needed.
• Add a view for managing file actions
  • Teachers can add and remove file actions from their course here.
  • Redirects to a new create/edit view
• Add a create/edit view.
  • If ?copy=1 is passed, it will create a copy of a file action.
• Changed file action parsing from self.command.split(" ") to shlex.split for more intuitive command behavior.
• Changes css rule .btn.btn-ion to .btn-ion
  • It was confusing if .btn did anything, or if you were supposed to do .btn.tin-btn, etc.

TODO

• CSS
• "Marketplace" for courses
• Views
• Docs
• Tests

Closes #7