Add endpoint to access user notifications observed by virtual user

[BenjaminPelletier]

This PR continues the work in #19 while shifting focus to the subject of the flight_planning API (virtual user).

Additional changes beyond #19's framework for setting up the user_notifications RESTful resource include:

• Updating API version to reflect addition of new functionality
• Updating description of permissions granted by interuss.flight_planning.plan
• Obtaining notification information from the virtual user rather than directly from the USS backend (since flight_planning is primarily an interface between the test director and USS representative/virtual user -- red shirt - green shirt -- and we want to avoid breaking system encapsulation as much as possible)
• Removing conflict_count since we separately are able to distinguish between unknown, 0, 1, and more than 1 conflicts and I'm not sure where (what kind of test) we would need to use conflict_count in addition to the conflicts enum (but we can add this back if we can identify a test like that and the requirement(s) it would be verifying)