return list of volunteers from playground projection

[dimitriirtimid]

Currently the projection only return the number of volunteers, it should return the complete list. Additionally the user profile projection should return all playgrounds that the user participates in.

DoD:

• [x] playground projection return the list of volunteers
• [x] implemented in the same fashion as the list of managers (so with object that contain both id and name)
• [x] the total volunteer count should remain part of the projection
• [ ] the Profile projection should include the list of playground ids that the user is participating in

[AnandAili]

The first three points of the Dod are implemented.