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/admin/invites/_invite.html.haml

@@ -0,0 +1,15 @@
+%tr
+  %td
+    .name-tag
+      = image_tag invite.user.account.avatar.url(:original), alt: '', width: 16, height: 16, class: 'avatar'
+      %span.username= invite.user.account.username
+  %td
+    = invite.uses
+    = " / #{invite.max_uses}" unless invite.max_uses.nil?
+  %td
+    - if invite.expires_at.nil?
+      ∞
+    - else
+      = l invite.expires_at
+  %td= table_link_to 'link', public_invite_url(invite_code: invite.code), public_invite_url(invite_code: invite.code)
+  %td= table_link_to 'times', t('invites.delete'), invite_path(invite), method: :delete if policy(invite).destroy?

app/views/admin/invites/index.html.haml

@@ -0,0 +1,22 @@
+- content_for :page_title do
+  = t('admin.invites.title')
+
+- if policy(:invite).create?
+  %p= t('invites.prompt')
+
+  = render 'invites/form'
+
+  %hr/
+
+%table.table
+  %thead
+    %tr
+      %th
+      %th= t('invites.table.uses')
+      %th= t('invites.table.expires_at')
+      %th
+      %th
+  %tbody
+    = render @invites
+
+= paginate @invites