Instructor Queue

This is an update that generally reflects the mockup posted at https://trello.com/c/YsbT00Ap/32-mockups

- instructor_id field was added to issues
- on the issues index view, new issue only appears for non-instructors (ie non-admin)
- instead, for admins, a list of issues added to their queue is displayed
- on the issue show view, admins/instructors see a button for adding the issue to their queue
- if they've already added the issue to their queue, the instead see a button for removing it.
- I also modified some labels, such as Archive issue rather than Resolve, since all issues that are archived are not necessarily resolved.
- includes an instructors array, which allows for signing in as a dummy instructor.  See https://trello.com/c/JkX8XdHK/64-sign-in-as-an-instructor
- includes some dummy issues added to seeds.rb