Prevent Facebook login if config is not set
Prevent Facebook login if config is not set

If the app_id or app_secret are not configured, then we cannot log in
via fFacebook, so exit early with a 501.

phoenixrises/joindin-api,lornajane/joindin-api,akrabat/joindin-api,akrabat/joindin-api,dstockto/joindin-api,joindin/joindin-api,heiglandreas/joindin-api,dstockto/joindin-api,lornajane/joindin-api,railto/joindin-api,lornajane/joindin-api,phoenixrises/joindin-api,joindin/joindin-api,asgrim/joindin-api,railto/joindin-api,phoenixrises/joindin-api,railto/joindin-api,heiglandreas/joindin-api,dstockto/joindin-api,asgrim/joindin-api,asgrim/joindin-api,joindin/joindin-api,heiglandreas/joindin-api,akrabat/joindin-api