Make sign up Magic Links work across devices

Previously if someone started signing in on their phone, but finished it on their laptop, they'd end up on the menu screen with no account. Bu tracking the purpose of a Magic Link we can always direct the user to sign up if they requested a magic link through sign up. This also makes the logic for changing the copy in the email more robust.

app/models/identity.rb

@@ -12,8 +12,10 @@ class Identity < ApplicationRecord
 
   normalizes :email_address, with: ->(value) { value.strip.downcase.presence }
 
-  def send_magic_link
-    magic_links.create!.tap do |magic_link|
+  def send_magic_link(**attributes)
+    attributes[:purpose] = attributes.delete(:for) if attributes.key?(:for)
+
+    magic_links.create!(attributes).tap do |magic_link|
       MagicLinkMailer.sign_in_instructions(magic_link).deliver_later
     end
   end