Allow management of Managers of Projects from Group API

[knikolla]

Currently it is not possible to add someone with a manager role through the API, so we've had to do this manually for TAs of a class.

There are two design issues to work out

• Groups currently represent users of a Resource Allocation, whereas the Manager role is on the Project rather than Resource Allocations.
• We're not exposing Projects through the API.

One possible solutions is to expose both memberships through the API as different groups:

Ex.
/Groups/allocation-123 would return the users of Resource Allocation 123 (currently /Groups/123).
/Groups/project-1234-users would return the users with a User role on Project 123.
/Groups/project-1234-managers would return the users with a Manager role on Project 123.