Fix comment schema; return error when response data doesn't match schema #1023

In #955, @rmccue was able to change the response format for a comment without updating the schema. The API shouldn't ever return a response that doesn't match the declared schema.

In this issue, we should:

1. Fix the schema, somewhat contingent on Establish criteria for when data should be nested in sub-objects on resources #1022
2. Return a 500 error when the produced response doesn't match the schema, instead of a malformed object.

Note for the second, I'm just referring to fields in the response that are missing or of a different format than in the declared schema. Additional fields can be ignored.