Create form to request access

Created by: wasabideveloper

Estimated duration: ~1.5d

See #182 (closed)

https://github.com/geometalab/osmaxx/commit/c4817c37c49128393077fb8b2e16111c8231b795#commitcomment-13307969

Replace the user_manager_username by a group. Prompt this group assignment on creation of the superuser.