Hard code receivers/simple to /services

The `receivers/simple` endpoint does not actually respect namespaces,
and if you try and access it at a particular namespace, it will not
work.

This change simply hard codes it to only ever go to
`/services/receivers/simple`, which will always work and do the right
thing.
Add back general JQuery HTTP implementation

With CORS support in Splunk, we can finally use regular `$.ajax`
requests when talking to Splunk. This adds back our old JQueryHttp
implementation.