Make the paste "Create" transaction take a file PHID instead of content

Summary:
Ref T4814. Although this approach made sense at one point, we have more file infrastructure now and T4814 will be easier if we just pass a PHID in.

Also swap Conduit over to use the Editor.

Test Plan:
  - Created a paste.
  - Created a paste via Conduit.
  - Verified that files had correct permissions and appropriate object links in Files.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4814

Differential Revision: https://secure.phabricator.com/D8969