Moved User.toString to Utils so it can be used elsewhere

`User.toString()` was formerly private and would check to see if a
`JSONObject` was null before we convert it to a `String`

I moved the funciton to the `Utils` class and made it public so that we
can construct `Submission`s with a `JSONObjec`t (something that we do over
and over again in User.java)

Perhaps we should also change the name of the function from `toString()` to something that makes more sense, maybe `JSONToString()` or something similar.