Resolve "Indicate head TAs"

You can now indicate head TAs for a course edition.

• A badge appears next to the name of the student head TA
• Buttons to promote and demote a student to head TA and TA
• Status changes to "Promoted" instead of "Accepted" (but promotion can only happen if student has accepted the offer)
• Experience table includes when a person was a Head TA for a course edition Closes #186 (closed)