releases as a dynamic view on the database was misguided, since
doing thing like adding a new job to a release set will invalidate
all old releases. So we rename release sets to views, and we'll
reintroduce releases as separate, static entities in the database.
the derivations that the jobset currently contains. This is
necessary to allow the "latest" channel to contain the correct
builds when the sources of a jobset are reverted.