* Invites: Add '1 week' as expire option

IMO a max. of 1 day is too short. Not everyone has the time and motivation to use an invite in a 24 hour period. 1 week as a max. is I think a good compromise between convenience and security.

* Invites: Add '1 week' as expire option

IMO a max. of 1 day is too short. Not everyone has the time and motivation to use an invite in a 24 hour period. 1 week as a max. is I think a good compromise between convenience and security.

* Update en.yml

* 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