Add consumable invites (#5814)

* 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

config/locales/simple_form.en.yml

@@ -30,10 +30,12 @@ en:
         data: Data
         display_name: Display name
         email: E-mail address
+        expires_in: Expire after
         filtered_languages: Filtered languages
         header: Header
         locale: Language
         locked: Lock account
+        max_uses: Max number of uses
         new_password: New password
         note: Bio
         otp_attempt: Two-factor code