Power Pack is now a key component of Review Board, and we want to enable

it by default just like we do with rbintegrations. This change adds it

to the auto-enable list so new installs will have it by default.
Older installs may have once had it but have disabled it. For these

installs, we'll need to auto-enable it on upgrade.
The Power Pack dependency and auto-enable in the Dockerfile has also been

removed from the Docker image, since that's taken care of now by the package

dependency and rb-site.

Created a local install and verified Power Pack was enabled. Disabled it

and upgraded, and verified it was re-enabled.
Conducted the same test within Docker using production-built packages.

Same results.