NL: misc fulfillment fixes

[pradh]

1 - Handle mixed fallbacks better

If we retrieve a place or variable from context and also do place fallback, provide a unified message.

Before this PR we would not report place fallback when we fallback from a place in context. [asthma in california] -> [what about blood pressure?]

Now we'll say, Tried looking up relevant statistics for "California" based on your prior queries, but found no results. Here are results for "United States" instead.

2 - Avoid duplicate SVs in comparison bar charts

[How long do workers in Maryland live] from earlier today

3 - Avoid bar charts that have no SV in title

These come about because we were expanding a "main SV" even when it doesn't pass existing check, so fix that.

4 - Add fallback info debug page

5 - Exclude more per-capita SVs

This is a temporary thing until we have chart controls for per-capita.