Readable Member Count

Adds `$readable` parameter to `dosomething_user_get_member_count` function to optionally return a human readable string like "3.1 million".

* Adds it as a token, closing https://jira.dosomething.org/browse/DS-458

* Outputs it into the Campaign pitch page if set, closing https://jira.dosomething.org/browse/DS-455