Add endpoint to GET specific users, restricted only to admins

## Add endpoint to GET specific users, restricted only to admins

### Acceptance Criteria
1. `GET` endpoint exists at `/users/:userId` where a user with the admin role (and no one else) can retrieve a user's information.
1. The endpoint does not return a user's password.

### Tasks
- Use the already-created `UserController`, but refactor the `get` method to be used for both `/user` (Get own user data) and `/users/:userId`.
- Use a converter object to retrieve the user.
- Create an `access_rule` to prevent all users except `ROLE_ADMIN`s from accessing the "get other users' data" endpoint. (Use a `RequestMatcher` to restrict `GET` requests at that endpoint.)
 - See http://silex.sensiolabs.org/doc/providers/security.html#defining-access-rules
 - See https://github.com/synapsestudios/synapse-base/blob/master/src/Synapse/Application/Services.php#L83

### Additional Notes
- None