New session length configuration settings

Added SESSION_COOKIE_AGE to mirror Django's but with 3 week expire time which allauth
has traditionally had.

Added SESSION_EXPIRE_AT_BROWSER_CLOSE mirroring Django's to always expire at browser close
no matter what expire time may be set.

Added ALWAYS_REMEMBER_SESSION setting to always remmeber the session rather than
requiring users to check a box on a form.