Rubrics init / wipe

Fixes #2914 (closed).

TODO:

• can init
• can wipe
  • provide info about how much damage you're doing and require confirming
    • ...by typing the shortname in a box
  • what are you supposed to do when the form does not validate POST data? namely when user does not provide shortname?
  • how to handle success deletion? A message and a button back to rubrics landing probably. redirected to main rubrics page
• cleanup
• what is the "username" we need to provide for init-ing rubrics? How do I get my own username? Or is this just some useless option that the command-line args need ("python manage.py plom_rubrics init manager")
  • todo: for now, just grabbed the .first() manager account: not sure it really matters as its just for the owner field of each autogen rubric.