Enable modules and openmrs-core to advertise capability or functionality

Lifted from https://talk.openmrs.org/t/how-to-make-the-reference-application-more-configurable/7632/29

I think the "better" long-term alternative is to explicitly have a way for modules (or system configurations) to register their capabilities (e.g. "registrationapp/registerPatient.page?appId=referenceapplication.registrationapp.registerPatient has the 'Register Patient' capability") and for the implementation to indicate which is its preferred url for each capability. This may be overkill. This could be implemented within the appframework module by using:

• Extension Points for "Register Patient capability"
• Extension to say this URL has "Register Patient capability"
• A new appframework feature to record which is the preferred URL for each capability.

Earlier in this thread, @gutkowski did a bunch of work on enabling "page overrides". I only just now peeked at this code, and at first glance I'm concerned that this implementation is specifically in the uiframework module, and wouldn't solve the problem when we're mixing uiframework-backed and OWA-backed pages. So (I think) this would need to be rewritten if it's to be the way going forwards. (It may be independently useful on its own for some other use cases.)