- Added a bunch of methods to BaseExercise that are related to the max_submissions and MaxSubmissionsRuleDeviation.
- Rewrote the parts of the BaseExercise.is_submission_allowed that considered the submissions_left check.
- Also some PEP8 refactoring near the changed and new code.
- Removed some unnecessary imports from submission_models.
- Added MaxSubmissionsRuleDeviationAdmin.