Adjust types and limits for common_grading/models.py

* Reduced MAX_LENGTH to 255 for varchar fields because that's the
  largest size with which we're guaranteed to be able to make an
  index on InnoDB with UTF-8 encoded text.
* Increased ESSAY_BODY_MAX_LENGTH to 10000 in response to limits
  discussion thread.
* Changed a couple of fields to text from varchar, just in case
  we ever decide to increase the size limits on them in the future.
Merge pull request #6 from edx/model_limits

Adjust types and limits for common_grading/models.py