Extensions: update "module" active for network function name to "extension"

[JDGrimes]

This is a function that is fairly likely to be used by extensions, so it would be nice to go ahead and update the name. We'll deprecate the current version (is_wordpoints_module_active_for_network()), and replace it with a new one named is_wordpoints_extension_active_for_network(). The code of the old one can be copied to the new one, and then the old one can be updated to just call the new one.

See #668.