hypothesis/client#603 broke the login popup window in Firefox and IE
because the call to `window.open` no longer happens in the same turn of
the event loop as the user's click on the "Login" link. It is therefore
no longer considered in FF to have happened "in response to a user
gesture".

This PR fixes the issue by separating creation and use of the popup
window into separate functions and moving creation to happen earlier, in
the same event loop turn as the "Login" button click.

Fixes #534