Add "why do you want to join" field to invite requests (#10524)

* Add "why do you want to join" field to invite requests

Fix #10512

* Remove unused translations

* Fix broken registrations when no invite request text is submitted

db/migrate/20190409054914_create_user_invite_requests.rb

@@ -0,0 +1,10 @@
+class CreateUserInviteRequests < ActiveRecord::Migration[5.2]
+  def change
+    create_table :user_invite_requests do |t|
+      t.belongs_to :user, foreign_key: { on_delete: :cascade }
+      t.text :text
+
+      t.timestamps
+    end
+  end
+end