Feature/update confirmation email information (#145)

* Add `invited_by` foreign key to User

Allows for a User to be tied to the user who invited them

* Include `current_user` in new agent initialization parameters

* Add `shoulda-matchers` for testing associations

* Add Inviter information and associated account to welcome email

* Only show inviter info if applicable

* Update conversation spec for FFaker compatibility

db/migrate/20191014051743_add_invited_by_to_user.rb

@@ -0,0 +1,5 @@
+class AddInvitedByToUser < ActiveRecord::Migration[6.1]
+  def change
+    add_reference(:users, :inviter, foreign_key: { to_table: :users })
+  end
+end