Add an endpoint for syncing a user's course status metadata

JIRA: MA-75

proposed path: users/{user}/course_status_info/{course_id}


Some things I am looking for feedback on:
1. Name of the endpoint. I've been calling it course_status_info but maybe metadata is a better word?
2.
  - Arguments. Name: last_visited_module_id.
  - Format: Right now I have it as form encoded on the setter. Should the body be JSON?

Also adds an "id" field to the path returned as part of the video_outline.

@gwprice @nasthagiri 
Note that this does not include combining the getter here with the video_outline endpoint into a new endpoint as we discussed. I figured we needed to have the symmetric getter for the setter no matter what and that would unblock Clarice. I'm happy to combine those into a new endpoint if we still think it's a good idea.

I figured we could make modification date be optional as a way to force a change, which seems better than getting someone to send a date in the arbitrary future. I could be talked out of it.

While I'm surely ignorant of some general performance implications here, I don't see how making it optional would slow things down. The fast path is when it's not present.