Hide the option to sort mappings if the site has no hits

This actually queries DailyHitTotals to determine whether the site
has hits, but we should assume here that that table is up to date
because:
- the hits import should always happen overnight so it's unlikely that
any discrepancy would exist when anyone is using the app
- hit percentages would be wrong anyway if DailyHitTotals isn't up to date