self-enrollment API, fixes CNVS-10419

move existing self-enrollment endpoint (undocumented) into enrollment api

depends on g/28504

test plan:
1. specs should pass
2. existing self enrollment forms should work, specifically:
   1. enrollment dialog on public course page
   2. self enrollment url (from course settings)
   3. canvas.net enrollment dialog

Change-Id: I0c876720203654e0a589b9ef2d25b80c3e4dfb08
Reviewed-on: https://gerrit.instructure.com/28506
Reviewed-by: Dan Dorman <ddorman@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>