Add "Remember me" option for login

[Eckankar]

See:

• http://mojolicious.org/perldoc/Mojolicious/Controller#session (current session expiration)
• http://mojolicious.org/perldoc/Mojolicious/Sessions#default_expiration (default session expiration)

We probably want to bump the default expiration a bit, it seems a bit short. With remember me, the session should be set to last a while. (3 months? More?)