Ensure login messages take you to the correct place after login

[TristanThrush]

There are a few places where dynabench asks you to log in if you aren't already logged in: when you go out of sandbox in the create interface, when you try to validate examples, when you try to submit predictions or train files. We should ensure that, when you log in via these messages, you are taken to e.g., the correct submit interface if you received the message in the submit interface.