(CE-755) Disallow login via FBConnect for disabled accounts

[CE-755](https://wikia-inc.atlassian.net/browse/CE-755)

This stops users from logging in to disabled accounts if they had
connected them to Facebook. Additionally, future account closures
will disconnect the account from Facebook.

This also provides a hook that will allow extensions to prevent a login
via FBConnect and display an error message. This is needed for CloseMyAccount.

@nandy-andy