The site-wide social login system is now backwards-compatible with how
the social_login_active core pref worked since commit
3b2d8333b62290688f64e061023669463fd8f04c.
social_login_active's least significant bit is now treated as a global
bit, so if it's not set, no other bits are allowed to be set. This
un-breaks all existing checks for whether social_login_active is empty.
Except in themes, the social_login_active check has been replaced with
an e_user_provider API to check if social login is enabled site-wide.
379a963902