GRN2-6: Added the ability for admins to specify registration method (#520)

* Added the ability to invite users

* Small bug fix

* Added the ability to approve/decline users

* Small bug fixes

* More bug fixes

* More minor changes

* Final changes

db/migrate/20190507190710_create_invitations.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class CreateInvitations < ActiveRecord::Migration[5.0]
+  def change
+    create_table :invitations do |t|
+      t.string "email", null: false
+      t.string "provider", null: false
+      t.string "invite_token"
+      t.timestamps
+    end
+  end
+end