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/policies/invite_policy.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+class InvitePolicy < ApplicationPolicy
+  def index?
+    staff?
+  end
+
+  def create?
+    min_required_role?
+  end
+
+  def destroy?
+    owner? || staff?
+  end
+
+  private
+
+  def owner?
+    record.user_id == current_user&.id
+  end
+
+  def min_required_role?
+    current_user&.role?(Setting.min_invite_role)
+  end
+end