Constrain givers and receivers to > 0; #1906
Back out fix for #1906; add a failing test instead

I don't like testing HTML output, but the communities listing needs to
be refactored and/or Aspen needs finer-grained hooks (so we can access
the receivers generator, before it's spent by the template section), and
that's too deep for now.
Fix #1906
Add a complementary test for givers and zero

This is the complement of the failure on #1906.
Merge pull request #1908 from gittip/receivers-receive

Constrain givers and receivers to > 0; #1906