Added customQuery to handle subscribed aggregates
Added customQuery to handle subscribed aggregates

refs https://github.com/TryGhost/Ghost/issues/12602

In order to build up a list of un/subscribes over time we have to use
the customQuery functionality to run SQL aggregates - this is "hidden"
behind an option, so that we can find{All,Page} as usual.

e10/Ghost,bitjson/Ghost,TryGhost/Ghost,ErisDS/Ghost,janvt/Ghost,TryGhost/Ghost,e10/Ghost,ErisDS/Ghost,janvt/Ghost,ErisDS/Ghost,bitjson/Ghost,TryGhost/Ghost