Add `fetchJobTasks` operation to the Jobs API. This returns a summary of
each of the job's tasks, suitable for display in a task list view.
The actually used fields may need tweaking once we actually have a task
list view, but at least the functionality is there.
- Addon switches between filesystem-packing and Shaman-packing
automatically, depending on whether the Manager has Shaman enabled.
- Actually using BAT for Shaman packing.
It doesn't work though, some error occurs when receiving Shaman response
from the Manager in the Addon.
This introduces some more conceptual changes to Shaman. The most important
one is that there is no longer a "checkout ID", but a "checkout path".
The Shaman client can request any subpath of the checkout directory,
so that it can handle things like project- or scene-specific prefixes.
This also requires that `TaskID` is no longer a custom type, because that
would cause import cycles. The alternative would be to put the generated
mocks directly into the `worker` package, but I didn't think that was
particularly nice. Maybe this'll be reconsidered later.
