Infinite redirect loop with prompt=select_account

[stefanfoulis]

related to #187

The only difference between prompt=login and prompt=select_account seems to be that with prompt=login the prompt GET parameter is removed from next_page before redirecting to the login page.
It looks like because it is not removed with prompt=select_account we end up with an infinite loop.

Probably we should just make prompt=select_acconunt behave the same as prompt=login.

To properly support prompt=select_account in the future, we'd probably need to provide some sort of hook so that developers can add logic to handle this special case correctly on the login page 🤔 . Perhaps pass the prompt=select_account GET parameter to the login page itself, instead of in next_page.