add control of self-enrollment type thru admin ui

fixes CNVS-10911

test plan:
- Navigate to /accounts/:id_of_root_acct/settings
- Toggle the 'Self Registration' feature -- when its checked, you should
  get a radio group to pick "All Account Types" or "Observer Accounts Only"
- In another tab, navigate to /login. It should render as follows:
  - Self Registration disabled : no banner
  - Self Registration enabled and
    - All Account Types chosen : "Need a Canvas Account?" banner
    - Observer Accounts Only chosen : "Parent of a Canvas User?" banner

Change-Id: Ie14e611f19afc00dfe363e765972b9959a2a34b0
Reviewed-on: https://gerrit.instructure.com/30620
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeff Belser <jbelser@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>