Implement views for file actions

[JasonGrace2282]

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.
• 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

[JasonGrace2282]

This PR is now ready to be tested and reviewed!
About documenting the usage of FileAction, I would prefer to merge #72 before writing docs about file actions here.