Implements Client Management Endpoints

We already had the R of CRUD, this adds the other 3. The 3 new endpoints require an oauth token with the scope `oauth`. Yo dawg, I heard you like oauth?

@rfk r?
@vladikoff you should take a look and see if there's something I've forgotten that the console will need.

fixes #60 
I assume `AUTH_SCOPE` is "the scope for permission to manage OAuth configs". It's probably a mixture of concerns to have it defined in this file, which implements how requests are authenticated. If we had a list of permissible scopes, it'd put it there. Also, `AUTH_SCOPE` alone doesn't signal to me what this is for. Auth scope for what? Something along the lines of  `OAUTH_MANAGEMENT_SCOPE` would be clearer to me, but I'm open to other options.