rbintegrations and Power Pack are both core to the Review Board

experience now, and will be required for any production install. We

tried to make sure these were enabled on upgrade to avoid issues with

older Power Pack trials that were since disabled, but the approach was

not ideal.
We now have formal support for always-enabled extensions in Djblets.

This change rips out the attempt we shipped for this in Review Board 8

and replaces it with a set of IDs that must always be enabled.

Tested that both rbintegrations and Power Pack were enabled at startup,

regardless of in-database state, and that they could not be disabled.