Many small fixes to the DB classes

Mostly comments and PEP8, but also some more serious things.
Small fixes to CWS
Remove cms/grading/AbstractionLayer.py

It was completely unused.
Some changes to ScoringService

- rename the 'submission_ids_*' sets to 'submissions_*'
- drop the token timestamp (and use just the submission id) inside the
  tuples that form the elements of the 'submissions_to_token' set
- don't use lists only to convert them to sets at the end, but start
  with sets from the beginning
- use 'get_from_id' instead of making a manual query
- some whitespace
Improve the 'invalidate_submission' methods of ES and SS

Mostly by rewriting get_submissions to return objects instead of IDs.
Reduce the use of export_to_dict and import_from_dict