Visual rearrangement of new dash "Student Admin" page
Enable use of student usernames on student admin page

This is in addition to email addresses, which also work.
Fix, and add more, tests
Enable alerts when a task succeeds.

Note: Alerts are pop-up boxes. Not sure if this is desireable, but I couldn't figure out
how to make a success message show up in the same place that error messages do.
Add more feedback for instructors on Student Admin section of the dash.
Merge pull request #1155 from edx/sarina/ins-dash-student-admin-2