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/_form.html.haml

@@ -0,0 +1,9 @@
+= simple_form_for(@invite) do |f|
+  = render 'shared/error_messages', object: @invite
+
+  .fields-group
+    = f.input :max_uses, wrapper: :with_label, collection: [1, 5, 10, 25, 50, 100], label_method: lambda { |num| I18n.t('invites.max_uses', count: num) }, prompt: I18n.t('invites.max_uses_prompt')
+    = f.input :expires_in, wrapper: :with_label, collection: [30.minutes, 1.hour, 6.hours, 12.hours, 1.day].map(&:to_i), label_method: lambda { |i| I18n.t("invites.expires_in.#{i}") }, prompt: I18n.t('invites.expires_in_prompt')
+
+  .actions
+    = f.button :button, t('invites.generate'), type: :submit

app/views/invites/_invite.html.haml

@@ -0,0 +1,11 @@
+%tr
+  %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/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