Validate email before creating identity during join code redemption

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.

See similar change in #1996 for sign-in and sign-up

app/controllers/join_codes_controller.rb

@@ -3,6 +3,7 @@ class JoinCodesController < ApplicationController
 
   before_action :set_join_code
   before_action :ensure_join_code_is_valid
+  before_action :set_identity, only: :create
 
   layout "public"
 
@@ -10,25 +11,35 @@ class JoinCodesController < ApplicationController
   end
 
   def create
-    identity = Identity.find_or_create_by!(email_address: params.expect(:email_address))
+    @join_code.redeem_if { |account| @identity.join(account) }
+    user = User.active.find_by!(account: @join_code.account, identity: @identity)
 
-    @join_code.redeem_if { |account| identity.join(account) }
-    user = User.active.find_by!(account: @join_code.account, identity: identity)
-
-    if identity == Current.identity && user.setup?
+    if @identity == Current.identity && user.setup?
       redirect_to landing_url(script_name: @join_code.account.slug)
-    elsif identity == Current.identity
+    elsif @identity == Current.identity
       redirect_to new_users_verification_url(script_name: @join_code.account.slug)
     else
       terminate_session if Current.identity
 
       redirect_to_session_magic_link \
-        identity.send_magic_link,
+        @identity.send_magic_link,
         return_to: new_users_verification_url(script_name: @join_code.account.slug)
     end
   end
 
   private
+    def set_identity
+      @identity = Identity.find_or_initialize_by(email_address: params.expect(:email_address))
+
+      if @identity.new_record?
+        if @identity.invalid?
+          head :unprocessable_entity
+        else
+          @identity.save!
+        end
+      end
+    end
+
     def set_join_code
       @join_code ||= Account::JoinCode.find_by(code: params.expect(:code), account: Current.account)
     end