Previously we scheduled a worker to just some this but we were running
into performance issues when the build table was getting too large. So
now we've updated the code such that this column is updated immediately
and incremented/decremented by the correct amount whenever artifacts are
created or deleted.
We've also added the performance optimization that we do not update this
statistic if a project is deleted because it could result in many
updates for a project with many builds.
Two things at ones, as there was no clean way to seperate the commit and
give me feedback from the tests.
But the model Artifact is now JobArtifact, and the table does not have a
type anymore, but the metadata is now its own model:
Ci::JobArtifactMetadata.
