Member count

@sergii-tkachenko Can you please review?

## New stuff:
Adds get/set methods for a Member Count variable, which will be accessible via the DS API.
* Closes https://jira.dosomething.org/browse/DS-427

## Cleanup:
Moves custom Services resource definitions out of `dosomething_campaign` and `dosomething_user` into `dosomething_api` -- to keep all Services related code in one module.
Will fix this in a later PR.