Removed obsolete references to sre_constants.error and replaced with re.error.
Clarified the names of some models
Separated the feedback system into its own app.
Added a better datetime notation into the template.
Some placeholders for better statistics.
Todos.
Clarified content graph name.
Lecture pages don't have form buttons anymore.
Some initial work on file checking.
Heavy work on the file exercise checking tasks.
Some more work on file exercise evaluation. Added a progress view for file exercise evaluation jobs.