implement rememberme functionality

implement rememberme functionality

Added a checkbox on login or register to remember the current user. If
the login is successful, this sets a cookie with a random code (saved
in the DB). If they come back, and they aren't logged in "normally",
we check to see if they have a rememberme cookie. If so, we log them
in.

However, they can't change settings -- cookie theft is too prevalent.
So we mark a session as having a "real" (password or OpenID) login, or
not. In settings pages, we check to see if the login is "real", and if
not, we redirect to the login page.

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