Individual UIDs for quizzes, worksheets and exams + removed disputing

The UIDs will be needed as part of the TeX compilation redesign

Taking away ability to dispute grade  from students for 3 reasons:
  1. there is nothing to dispute as they don't get to see grades anyway
  2. its never been used while it was there
  3. need to see a clearer model before going into further rewiring
G.Response -> Worksheet -> Exam -> Quiz
G.Response -> Worksheet -> Exam -> Quiz
Removed 'compiled' column from Worksheets

What we need instead is a compilation_required? method
Worksheet now :belongs_to :quiz also

Worksheets made in an institutional environment have
both exam_id and quiz_id set

But those made in the online offering have just quiz_id set ( exam_id = nil )
This is how we will differentiate between worksheets
Edited / defined some obvious query methods
Flip-flopping: Worksheets -> Quiz via Exam only ...

Exam as a model has its purposes - albeit in an institutional environment only.
However, its there and rather than bypass it, its behaviour in an online
envrionment must be defined explicitly
 New method to find (last) sektion common to a group of students
Method common_to returns the whole lit of common sektions - order by date of creation
Added 'business_time' gem to do deadline calculations ...
Sending mails to graders and teachers after scans distributed
Changed image to show printer for option to print composite exam
Humungous changes - too many to list individually ...

But basically, there is now a new mint/ structure and a new QR code
scheme
Detaching quick trial demos from teacher console ...

Commented out - but not removed - the code written previously for demos.
T'was a good idea and elements of it might come in handy later. Hence,
retaining the code for future reference - even if not using it now
Some more code pertaining to quick trial demos detached
Shadow calculated during layout and stored in the quiz itself
Track whether or not the scan for a response is a mobile upload or not ...

Also, set the shadow to 0 for mobile scans
More \'s needed to truly escape latex name
Student inbox now points to new paths in mint/
Merge branch 'rewire'

Conflicts:
	config/routes.rb