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

app/views/invites/index.html.haml

@@ -0,0 +1,19 @@
+- content_for :page_title do
+  = t('invites.title')
+
+- if policy(:invite).create?
+  %p= t('invites.prompt')
+
+  = render 'form'
+
+  %hr/
+
+%table.table
+  %thead
+    %tr
+      %th= t('invites.table.uses')
+      %th= t('invites.table.expires_at')
+      %th
+      %th
+  %tbody
+    = render @invites