Remove Files Widget from Conpherence

Summary: Fixes T8834. Removes everywhere I could find references to Files.

Test Plan: Use Conpherence, send a message, attach a file, try durable column, send message, send file. Seems snappy.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T8834

Differential Revision: https://secure.phabricator.com/D13936