Make the id property available on both modules and descriptors, and make it a property, so that we aren't duplicating the location data

Move field definitions for XModules into mixin classes, so that descriptors and modules always have the same set of fields

Make default value for format setting None, and force lms and cms to be explicit about a string default

NumericResponse was failing to accept as correct an answer with a plus
sign in the exponential. For example: 5.0e+1 was marked as incorrect
if the answer was 50. However, 5.0e1 was marked as correct. This
caused confusion, since the answer shown to students included the plus
sign.

LMS Lighthouse [#242]

with a custom message for when they submit something.