Add user search

[jjnesbitt]

Prerequisite functionality for multinet-app/multinet-client#107

Could be something like GET /api/user/search, which would perform a prefix search on the users collection. The fields to search which initially seem the most useful are email and name. We might want a way to specify the field to search as a query param on the endpoint, returning a 4XX error if the specified field doesn't exist, or isn't allowed to be searched on.