Handled warning about PNG already being a registered MIME type. Removed other types that are already commented out. Removed unnecessary comments.
ASSIST-80 - Reorganized top of User model.
ASSIST-80 More focused tests for KeyPerson

- Still unclear if :user and :submission can be nil and the record still
  be valid. I found cases of both of these being nil in production, and
  sent Paul and email asking about it.
ASSIST-80 Cleaned up tests on two models. Reorganized things to refledct the structure of the model code, to establish a consistent hierarchy in model code, and to make it more clear in the tests which scenarios are covered by the tests, and which aren't.
Merge branch 'master' into test-suite-update
Merge branch 'master' into test-suite-update
ASSIST-80 Corrected README