prompt the user if they try leaving the page before confirming their tip
disable the tip-changed prompt when trying to sign in

also disable when the tip value is reset
remember the tip entered through the sign in process
update code after rebase

we used to use <a> links for signing in, but @Changaco turned them into
POST forms for enhanced security
fix bug found by @rummik

Specifically:
https://github.com/gittip/www.gittip.com/pull/2039#discussion_r10637514

Don't know how it was working for me without this… o_0