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

db/migrate/20171125024930_create_invites.rb

@@ -0,0 +1,15 @@
+class CreateInvites < ActiveRecord::Migration[5.1]
+  def change
+    create_table :invites do |t|
+      t.belongs_to :user, foreign_key: { on_delete: :cascade }
+      t.string :code, null: false, default: ''
+      t.datetime :expires_at, null: true, default: nil
+      t.integer :max_uses, null: true, default: nil
+      t.integer :uses, null: false, default: 0
+
+      t.timestamps
+    end
+
+    add_index :invites, :code, unique: true
+  end
+end

db/migrate/20171125031751_add_invite_id_to_users.rb

@@ -0,0 +1,5 @@
+class AddInviteIdToUsers < ActiveRecord::Migration[5.1]
+  def change
+    add_reference :users, :invite, null: true, default: nil, foreign_key: { on_delete: :nullify }, index: false
+  end
+end