Q&A answer handling: delete/edit/get/close

[snapcat]

If you decide to work on any of these, click "convert to issue" to on the right and assign yourself. Also leave a comment below.

For one question (will likely require a database table update):

• QA Command: delete a single answer from a question (Instructor only)
• QA Command: edit a single answer from a question (Instructor only)
• QA Command: get a single answer from a question (by what criteria? may not be necessary, see getAnswersFor command)
• QA Command: prevent any more answers from being added to a question. Instructor only (will require an is_closed boolean column in the questions table; see is_ghost handling in qanda.py for examples)

Other:

• QA Command: getAnonAnswersFor (q_num)
• QA Command: getStudentAnswersFor (q_num)
• QA Command: getInstructorAnswersFor (q_num)