Add sortedSetPush

[aghassemi]

In many cases, we want to have a set of namespace items or glob results.
See src/onboarding.js or src/services/namespace/service.js

sortedPush works well when the array is allowed to have repeats. We have implemented this already.
sortedSetPush would go a step farther and first ensure that the item to be added isn't already present in the array.

This would be nice to have, but we have workarounds in place already. Too bad there's no observ-set in Mercury.