FIX: display warning when SSO email is different from invite email (#13804)

In this commit, we skipped frontend validation when email is obfuscated:
https://github.com/discourse/discourse/commit/534008ba24c

However, if email from SSO is different from email from invite, we should still display warning.

app/controllers/invites_controller.rb

@@ -50,10 +50,13 @@ class InvitesController < ApplicationController
       email = Email.obfuscate(invite.email)
 
       # Show email if the user already authenticated their email
+      different_external_email = false
       if session[:authentication]
         auth_result = Auth::Result.from_session_data(session[:authentication], user: nil)
         if invite.email == auth_result.email
           email = invite.email
+        else
+          different_external_email = true
         end
       end
 
@@ -73,6 +76,10 @@ class InvitesController < ApplicationController
         email_verified_by_link: email_verified_by_link
       }
 
+      if different_external_email
+        info[:different_external_email] = true
+      end
+
       if staged_user = User.where(staged: true).with_email(invite.email).first
         info[:username] = staged_user.username
         info[:user_fields] = staged_user.user_fields