Some users still report slow queries. Investigation using the info in
"About > Database" shows the workers intended to prune the database and
keep query times reasonable aren't running.

Add an additional "About > Workers" tab with additional information. For
the three workers the current `WorkInfo` is displayed, as are the logs
for those workers, to help with debugging this.

To do this:

- Move the workers from `app` to `core.workers`, so they can be visible
to `feature.about`. This also moves their dependent code.
- Add additional logging to the workers, add their tags to the list of
tags logged to the database in `LogEntryTree`.
- Modify the DAO methods that prune the database to return the number of
affected rows.
- Extract `LogEntryAdapter` and `WorkInfoAdapter`.
- Include tags and periodicity information when displaying `WorkInfo`.
- Add a Fragment/ViewModel to display the worker information.
- `asDdHhMmSs()` now handles negative durations.
- Periodic work is enqueued with an `UPDATE` policy (was `KEEP`).