More tag-ification for open_class_category
Tag-ifies the open class category name/symbol
Moves open_class_category to be a property of Program

Since open_class_category is a program tag, it makes sense to make
it a property on a program. If there is no tag for the particular
program, it defaults to the global tag, which is the desired
behavior.
Adds a default for open_class_category that is not None
Removes qsd_tag and updates the open_class_category property
Merge pull request #1 from gkanwar/openclass_tag_fixes_gurtej

Open_class_category tag fixes

These commits add open_class_category as a program tag, accessible as
the `Program.open_class_category` property. Its value should be the
primary key of the intended ClassCategories instance. If None, it
defaults to finding or creating one named "Walk-in Activity" with
symbol "W".