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/controllers/sessions/magic_links_controller.rb

@@ -9,9 +9,9 @@ class Sessions::MagicLinksController < ApplicationController
   end
 
   def create
-    if identity = MagicLink.consume(code)
-      start_new_session_for identity
-      redirect_to after_authentication_url
+    if magic_link = MagicLink.consume(code)
+      start_new_session_for magic_link.identity
+      redirect_to after_sign_in_url(magic_link)
     else
       redirect_to session_magic_link_path, alert: "Try another code."
     end
@@ -21,4 +21,12 @@ class Sessions::MagicLinksController < ApplicationController
     def code
       params.expect(:code)
     end
+
+    def after_sign_in_url(magic_link)
+      if magic_link.for_sign_up?
+        new_signup_completion_path
+      else
+        after_authentication_url
+      end
+    end
 end