The Repository API has always been pretty limited, simply wrapping the

creation or editing of a Repository model instance. This meant that we

had no good way of supporting modern features like hosting services, and

we had to duplicate a lot of complicated logic and error checking.
This change updates the API to simply wrap RepositoryForm. This gives

us all the checking and capabilities of the form, and keeps the API's

code very small.
Right now, we still don't support anything advanced, like hosting

services, but that can now more easily come in future updates by simply

enabling the appropriate request fields in the API.

Unit tests pass.