Added plantings_count (i.e. counter_cache) to Member

Also removed the superfluous .plantings_count method on Crop, which was
causing some confusion.

A thing we learned today: we should use .size to find the size of
ActiveRecord collections, not .count, because .count doesn't use the
cache (while .size does).
Merge branch 'planting-owner' of github.com:Skud/growstuff into planting-owner