convert default_scope to use blocks

Rails 4.2 only supports `default_scope` with a block.  Rails 3.2 also
supports `default_scope` with a block, so this converts all of our
default scopes to use blocks.  This way our default scopes will work on
3.2 and 4.2

This seems to fail tests, but it looks like we're stubbing `where` on a model, and the default scope will also call `where`.  Can I get some help please?