Validate email before creating identity during sign-up and sign-in

Avoid Sentry exceptions when attackers try to stuff invalid emails. The
browser performs form field validation that should normally prevent this
from occurring, so we just return 422 without validation error messages.

Also:

- extract redirect_to_session_magic_link helper
- some controller refactoring and cleanup

app/controllers/join_codes_controller.rb

@@ -20,8 +20,11 @@ class JoinCodesController < ApplicationController
     elsif identity == Current.identity
       redirect_to new_users_verification_url(script_name: @join_code.account.slug)
     else
-      logout_and_send_new_magic_link(identity)
-      redirect_to session_magic_link_url(script_name: nil)
+      terminate_session if Current.identity
+
+      redirect_to_session_magic_link \
+        identity.send_magic_link,
+        return_to: new_users_verification_url(script_name: @join_code.account.slug)
     end
   end
 
@@ -37,13 +40,4 @@ class JoinCodesController < ApplicationController
         render :inactive, status: :gone
       end
     end
-
-    def logout_and_send_new_magic_link(identity)
-      terminate_session if Current.identity
-
-      magic_link = identity.send_magic_link
-      serve_development_magic_link(magic_link)
-
-      session[:return_to_after_authenticating] = new_users_verification_url(script_name: @join_code.account.slug)
-    end
 end