R1:c2f58496ad93

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

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