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/controllers/admin/invites_controller.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+module Admin
+  class InvitesController < BaseController
+    def index
+      authorize :invite, :index?
+
+      @invites = Invite.includes(user: :account).page(params[:page])
+      @invite  = Invite.new
+    end
+
+    def create
+      authorize :invite, :create?
+
+      @invite      = Invite.new(resource_params)
+      @invite.user = current_user
+
+      if @invite.save
+        redirect_to admin_invites_path
+      else
+        @invites = Invite.page(params[:page])
+        render :index
+      end
+    end
+
+    def destroy
+      @invite = Invite.find(params[:id])
+      authorize @invite, :destroy?
+      @invite.expire!
+      redirect_to admin_invites_path
+    end
+  end
+end