Rework /for/ to not talk about viability

Instead we just refer to "Large Communities" and "All Communities." Large is simply defined as the 18 largest (fits the 3-column listing well). Note that NMEMBERS_THRESHOLD survives, because I'm still thinking we'll vary what we show for > and < THRESHOLD (only the "New Members" listing below THRESHOLD; all three listings as usual for above).

This PR depends on #1908.