Fix improper selection of the chunk engine as a writable engine

Summary:
Fixes T7621. The engine selection code started out making sense, but didn't make as much sense by the time I was done with it.

Specifically, from the vanilla file upload, we may incorrectly try to write directly to the chunk storage engine. This is incorrect, and produces a confusing/bad error.

Make chunk storage engines explicit and don't try to do single-file one-shot writes to them.

Test Plan:

• Tried to upload a large file with vanilla uploader, got better error message.
• Uploaded small and large files with drag and drop.
• Viewed Files → Help/Options.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7621

Differential Revision: https://secure.phabricator.com/D12110