Following feedback on D12011, decided to refactor the aux user store actions.
In D12011, we want a single action type that both clears prior fids and adds payload fids.
As `setAuxUserStoreFIDs` has yet to be used anywhere in our codebase, I have the action type first clear fids and add payload fids.
The previous functionality of just adding fids becomes `addAuxUserFIDs`.