Common green line branch stops should report issues in other branches

[radhikamalik]

Branches have several stops in common. The stops should share issues.

[geoffreylitt]

@davidlago and I discussed this on Tuesday and we actually weren't sure what to do for these cases. I'm sure there are situations like a disabled train where branches affect each other--but maybe there are others, like "delayed train", where they don't?

In any case, the current technical implementation prevents us from doing any event sharing between lines, so I agree we should at least make it possible.

This might tie in well with the GTFS API integration -- I think that branches share stops in that data model.