MBS-357: Add a store for "remember me" tokens

This is for an implementation of http://fishbowl.pastiche.org/2004/01/19/persistent_login_cookie_best_practice/
MBS-357: Implement a new "remember me" system that doesn't involve passwords

Instead, we use random single-use tokens, in the approach proposed in http://fishbowl.pastiche.org/2004/01/19/persistent_login_cookie_best_practice/