EDA Frontend "Only show variables with data for this subset"

[danicahelb]

Related to: #724

Once backend is in place, implement frontend UI

User request an "audit" which variables "have at least one non-null value" for the current subset

This could be done with a toggle Only show variables with data for this subset, similar to the Only show compatible variables toggle on the viz tab var selector.

This toggle should appear on all variable trees (B&S and Viz tabs on EDA; Configure map & Filter on SAM)

@bobular: given the expense of the computation, maybe a one-off button rather than a toggle would be better