R1:92ea4fb098cb

Stop two special cache writes in read-only mode

Summary:
Ref T10769. The user availability cache write shouldn't happen in read-only mode, nor should the Differential parse cache write.

(We might want to turn off the availbility feature completely since it's potentially expensive if we can't cache it, but I think we're OK for now.)

Test Plan:
In read-only mode:

- Browsed as a user with an out-of-date availability cache.
- Loaded an older revision without cached parse data.

Reviewers: chad…