Need a way for apps to declare custom content types

[jbroadway]

Apps should be able to specify a conf/courses.php file that (among other possible future uses) declares custom content types provided by that app in the form of handlers or method calls.

An example would be the chat app providing a chat box that can be embedded into a course. An author would create a 'Learner Chat' page in their course, and add the 'Chat Box' custom content type as an item on that page. Learners could visit that page to participate in the chat.

Note that this is just an example. Chat may be something that gets more deeply integrated into the system at some point, but serves as a simple example for the need for this kind of integration.