R1:74d5ae7bec46

Clean up some log spam caused by races in VersionedDraft

Summary:
Two minor issues that I caught in the log while fixing Phame permissions:

- We had a JS bug which would cause us to immediately generate two comment previews at the exact same time -- one for loading the page, and one for "switching to desktop". Instead, only generate the "switch to desktop" preview if we really switched to desktop from a different device layout.
- These two requests could end up reading/writing the VersionedDraft…