R1:45b386596e62

Make the Files "TTL" API more structured

Summary:
Ref T11357. When creating a file, callers can currently specify a `ttl`. However, it isn't unambiguous what you're supposed to pass, and some callers get it wrong.

For example, to mean "this file expires in 60 minutes", you might pass either of these:

- `time() + phutil_units('60 minutes in seconds')`
- `phutil_units('60 minutes in seconds')`

The former means "60 minutes from now". The latter means "1 AM, January 1, 1970". In practice, because the…