Unify the descriptor/module system (Phase 2, xBlock conversion)

[e0d]

This work will unify the platform’s content descriptor system and module system:

• eliminating a historically common source of difficult to diagnose memory leaks,
• reducing redundant implementations of courseware services (e.g. course static asset linking).
• removing the single most confusing aspect of the XBlock runtime (descriptor vs. module system).

Customer Impact	Details
Goal	Development work done to modernize and simplify our core content middleware and delivery engine, migrating from our legacy xmodule system to our xblock infrastructure.
Reach (Customer)	Developers making improvements to Open edX’s core content middleware layers.
Impact + Measure (Outcome)	Simplified development timelines and improved diagnosis of content issues will be possible by simplifying platform areas that are often poorly or not understood by internal teams.

[kdmccormick]

Not sure if it fits into this phase, but here's one related item:

• Remove special XModule asset handling edx-platform#31624