META: Refactor all Assignment subtypes into a single activity module

The assignment module is a core part of Moodle that has evolved over a long time. Unfortunately it's a lot more complicated than it should be due to the old subtypes that were added.

To simplify the code and the interface, all of these subtypes can be replaced by one module that takes the following inputs:

• a HTML text (via editor)
• a set of files

and provides the following items for feedback:

• a HTML text (via editor)
• a set of files
• one or more grades (numerical or scales)

For the time being (2.1) we do not need to consider enhancements. This is just a refactor. All old assignments should upgrade cleanly to the new structure without losing any features.

This issue details the sub-tasks required to make this happen.