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