Refactor registry schemas

[jamesaoverton]

Building on #31, I'd like to merge these schemas into a single JSON Schema file:

https://github.com/OBOFoundry/OBOFoundry.github.io/tree/master/util/schema

In addition to reuse of "definitions" and "$ref", this will require the use of JSON Schema conditionals:

https://json-schema.org/understanding-json-schema/reference/conditionals.html

This is a bit of an experiment, but if successful we'll merge it into the registry repo. Since the dashboard uses a couple of the small schema files, we'll have to update it somehow.