Implement file actions management

[krishnans2006]

Tin supports file actions through assignments.models.FileAction and a corresponding view. The implementation runs user-defined scripts on the files uploaded to an assignment. It's main use case is for scripts that must be run on the Tin VM, like compiling code for a certain architecture and program version.

However, there is no way to create new file actions or edit existing ones (e.g. to add something to path or add a library). In such instances, a Tin Administrator is emailed the request, and they use the Django admin portal to make the change.

A way to manage file actions and the courses they are enabled for would be a nice feature. A button can be placed on the home page, next to the "Manage virtual environments" button, to accommodate this feature.