Turn on full file verification

[Jason Gerard DeRose]

In addition to running the scan and relink background tasks by default, we're also now going to turn on full-content-hash verification. We've had the functionality for quite a long time, but it's never been turned on by default.

Initially this is going to be very dumb: it will run whenever there are files that have gone more than 1 week without full verification (on the connected filestores, as that's the only thing it can verify, obviously). But this is something that eventually we'll want to be more selective with. For example, we probably don't want to run this when on battery power, or when importing, or when for other reasons the system load is high.

But all the same, having this running by default is needed to be production ready, and with time we can tune this to be more intelligent about when it runs.