Write "attach" edges when files are attached to objects via comment or other transactions

Summary: Ref T603. When a user comments on an object with an embedded file, write an "attached" edge.

Test Plan: Made a comment on a task with an embedded file, verified the edge was written in Files.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7191