R1:47dedfb152d0

Introduce "bridged" objects

Summary:
Ref T10537. These are objects which are bound to some external object, like a Maniphest task which is a representation of a GitHub issue.

This doesn't do much yet and may change, but my thinking is:

- I'm putting these on-object instead of on edges because I think we want to actively change the UI for them (e.g., clearly call out that the object is bridged) but don't want every page to need to do extra queries in the common case where zero bridged objects exist…