Eliminating various FXCop warnings, an extra reference to filedrop.js, and updating JSON response from images redir

There are various FXCop rules addressed in this update:
* Avoid usage of generic collections
* Prefer property { get; set; } pattern to class members
* Implement all of the exception interfaces in new exceptions
* Use static methods when there is no reason to use an object
* Remove unused references (done for Moment objects when writing App Activities)

In addition, this update addresses a change in the data returned from the api/images endpoint:
* JSON is returned instead of a string in the backend
* The services class for the frontend interprets the JSON instead of assuming a string