Add support in the API for admins to create repositories.

This provides HTTP POST/PUT/DELETE support for /api/repositories, allowing
administrators to create, modify, or "delete" (hide) repositories.

The API can return a number of errors, based on configuration problems or
host access problems (such as bad/unknown SSH keys or unknown certificates).
In the cases where it's host-related information that needs to be verified,
the API can respond to the error by repeating the request and sending
trust_host=1.

Unit tests were added to test creating, updating, and deleting repositories,
both on global sites and LocalSites, and testing the various error results.

All the unit tests pass.