AID-599: Redirect user back to URL when doing a POST

Nope, but we need to do this in two steps. The front-end currently expects this to return 200:
https://github.com/amedia/mercury/blob/a59abe85975d6cb9ac01bb9e7aa531b5b8c1f121/src/js/views/responsive/login/login.js#L38

So we first need to change the front-end to expect *both* 200 *and* 401, deploy it, and wait a day or two. (Since we don't have control over when people last downloaded the JS file to their computer).

THEN we can change the behaviour of this endpoint and return 401 Unauthorized. :8ball: