* clean up and document integration test convention
* clarify integration test conventions
* clean up integration tests that don't follow convention
* mark testIntegration* functions as helpers to avoid confusion
Removes various custom headers logic sprinkled around in the backend.
It should automatically be applied to outgoing HTTP requests via the
CustomHeadersMiddleware.
This also removes decryption of SecureJSONData to populate custom
headers in ngalert which seemed to have caused a ton of CPU usage.