Separate `Problem` object from its testdata

[kiritofeng]

There are instances of problems being cloned (primarily for teaching purposes) where only minor adjustments are made to the problem (e.g. minor changes to the statement, changes to TL/ML, changes to allowed languages, etc), however, these require the test data to be cloned as well.

I propose renaming the current Problem object to ProblemInstance, and creating a new ProblemManager object that is responsible for managing the test data:

• The ProblemInstance object would hold all of the current fields of Problem, but no longer be responsible for managing test data, instead it will have a ProblemManager ForeignKey
• The ProblemManager will have a code field, which is unique among all ProblemManager objects
• The problem OneToOne field in ProblemData will be changed to a ProblemManager.

When creating a problem, an option should be made available to select the desired ProblemManager, or to create a new one (with the same code as the ProblemInstance). When cloning a problem, an option should be given to also clone the test data, which corresponds to copying the ForeignKey of the ProblemManager of the problem being cloned.

When a problem's code is changed, if the associated ProblemManager shares the same code as the problem, its code should also be updated. This should ensure the code of a ProblemManager is one of the codes belonging to an assigned ProblemInstance object, modulo DB tampering.