CYB-270 Support for filter by assignments in the Gallery

Include assignment name API call

  * new Assignment model (so requires migration)
  * hooks into the assignments_processor to get assignment data
  * only adds one SQL query/type (image/video) per clock tick, plus one
      SQL write per assignment
  * action /index on assignments added

  NEW ROUTE:
  /api/v1/assignments

[CYB-270 Support for filter by assignments in the Gallery, include assignment name](https://jira.ets.berkeley.edu/jira/browse/CYB-270)