Add site connections endpoints

This adds support for the following REST API endpoints:

- GET `/site/$site/connections`
- GET `/site/$site/connections/$connection_id`
- POST `/sites/$site/connections/$connection_id/delete`

Tests could not be added for deleting connection since endpoints don't exist (yet) for creating a new connection.
@aduth ok .. I got an error when I run tests. I think we should remove tests and wait when the other endpoints are implemented.
Question: these endpoints are documented ?