Prompt user when leaving page w/ new tip & store tip through sign-in

This started out as an effort to resolve #1463, but I never actually got there.

+ prompts the user if they try leaving the page with an unconfirmed tip
+ for new users, store the entered tip so when they sign in their original tip value isn't lost

I'm going to say this is a work in progress, as I'd still like to fix #1463