Expose the "file attached to object" and "object attached to file" edges via "edge.search"

Summary:
See PHI1901. An install would like improved support for identifying files related to an object (like a task or revision) for retention/archival/backup/migration/snapshotting purposes.

The "attachment" edge is not really user-level: it just means "if you can see the object, that allows you to see the file". This set includes files that users may not think of as "attached", like thumbnails and internal objects which are attached for technical reasons.

However, this is generally an appropriate relationship to expose for retention purposes.

Test Plan: Used "edge.search" to find files attached to a revision and objects attached to a file.

Differential Revision: https://secure.phabricator.com/D21480