* Add consumable invites
* Add UI for generating invite codes
* Add tests
* Display max uses and expiration in invites table, delete invite
* Remove unused column and redundant validator
- Default follows not used, probably bad idea
- InviteCodeValidator is redundant because RegistrationsController
checks invite code validity
* Add admin setting to disable invites
* Add admin UI for invites, configurable role for invite creation
- Admin UI that lists everyone's invites, always available
- Admin setting min_invite_role to control who can invite people
- Non-admin invite UI only visible if users are allowed to
* Do not remove invites from database, expire them instantly
* Add moderator role and add pundit policies for admin actions
* Add rake task for turning user into mod and revoking it again
* Fix handling of unauthorized exception
* Deliver new report e-mails to staff, not just admins
* Add promote/demote to admin UI, hide some actions conditionally
* Fix unused i18n
