[api] Implement service webhooks

GitHub announced the deprecation of GitHub services in favor of
webhooks (on January 31, 2019 GitHub will stop
delivering services events).
WebHooks are designed different than services, therefore it was necessary to
implement a dedicated controller.
Unlike the OBS GitHub service, webhooks do not deliver the token anymore
in the HTTP Header. Because of that it is not possible to compare
the tokens anymore as part of the authentication process.

We used this opportunity to implement generic webhooks which support
for now GitHub, Pagure and a generic webhook.
To trigger a webhook, it is now necessary to generate a signature
over the payload body with the secret token.
To authorize, OBS verifies this signature.
OBS supports for now three different HTTP header to submit this signature:
* HTTP_X_OBS_SIGNATURE (generic OBS header)
* HTTP_X_HUB_SIGNATURE (GitHub)
* HTTP_X-Pagure-Signature-256 (Pagure)

It is now also necessary to send the token id with the webhook request.
This PR introduces the new route:
'trigger/service/github?id=42'

See these URLs for more details:
https://developer.github.com/changes/2018-04-25-github-services-deprecation/
https://developer.github.com/webhooks/securing/
https://docs.pagure.org/pagure/usage/using_webhooks.html