AbstractOAuth2Controller: Store provider and token in class properties
AbstractOAuth2Controller: Store provider and token in class properties

This way, they are available for subclasses to access them in one of
the template methods.

Refs #673.

flarum/core,Luceos/core,malayladu/core,renyuneyun/core,flarum/core,datitisev/core,datitisev/core,Luceos/core,Luceos/core,datitisev/core,malayladu/core,renyuneyun/core,malayladu/core,datitisev/core,Luceos/core,flarum/core,renyuneyun/core,malayladu/core,renyuneyun/core