fix the bug that the first caption and the second one will become current at the same time when displaying right captions
fix the bug: when click a caption, the previous one will be in bold
fix the video caption lag issue
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.