add support for self enrollment caps to limit the size of a particular
course. there is no UI for setting caps (yet)
test plan:
1. run specs
2. follow test plan for https://gerrit.instructure.com/15819
Change-Id: Ibf0a8f04f0c2efa820d0850cef26dfae20849246
Reviewed-on: https://gerrit.instructure.com/16021
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joe Tanner <joe@instructure.com>
QA-Review: Cam Theriault <cam@instructure.com>
Jon Jensen