Undo reversion of "Merge branch 'JetDecorators_190729' into 'master'"

This reverts merge request !30491 (merged), thus re-instating the contents of !25234 (merged). The original MR was reverted (temporarily, while we worked out a solution) because it was causing warnings in jet reconstruction. This turned out to be due to the fact that jet modifiers (some of which newly use DecorHandles) were being applied before the jet collection was recorded to the event store. That issue has since been fixed in !30581 (merged), so it should now be safe to put these changes back in.