Adds JSON module to all existing programs

Adds a South data migration to add the JSON Data Module to all existing
programs. The JSON Data Module is a dependency for many commonly-used
modules, so it is important that it be enabled for all programs.

The migration deletes all existing JSON ProgramModuleObjs, to avoid an
edge-case where duplicate JSON ProgramModuleObjs cause Django errors.
This is safe, because ProgramModuleObjs are automatically regenerated.
Always adds JSON module in new program setup

Adds some lines to the prepare_program setup function, to always include
the JSON Data Module in the list of selected modules. The JSON Data
Module is a dependency for many commonly-used modules, so it is
important that it be enabled for all programs.
Adds install() to add_json_module migration
Merge pull request #614 from learning-unlimited/enable_json_module

Enable json module