Redirect user to the page they initiated the login flow from

[mxk0]

Summary

Right now, when a user creates an account or logs in, we redirect them to simpler.grants.gov (the root). This ticket is to add handling to redirect the user to the page they initiated the create account/login flow.

This won't take other state into account (if the user had a modal open, if they were typing in a field, etc.). If a user kicks off the auth flow by trying to save an opportunity, they'll be redirected to the opp after auth and have to click "save" again.

Also, because auth is redirect based, using the "back" button won't get the user back to search results from an opportunity.

Acceptance criteria

• User is redirected to the page they initiated create account/login