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 anywhere in the system.
• I'm making these one-to-one, more or less: an issue can't be bridged to a bunch of tasks, nor can a bunch of tasks be bridged to a single issue. Pretty sure this makes sense? I can't come up with any reasonable, realistic cases where you want a single GitHub issue to publish to multiple different tasks in Maniphest.
• Technically, one type of each bridgable object could be bridged, but I expect this to never actually occur. Hopefully.

Test Plan: Ran storage upgrade, loaded some pages.

Reviewers: chad

Reviewed By: chad

Subscribers: Luke081515.2

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15502