webhook-service has to work with private repositories

[jachro]

This PR makes the webhook-service's endpoints to work with private repositories. The changes to the endpoints are:

• POST /projects/:id/webhooks/validation validates project's hook. The behaviour of the endpoint is the following:

Project visibility	Scenario	Response code	Notes
Public	Hook exists	OK (200)
Public	Hook does not exist	NOT_FOUND (404)
Private	The given Access Token is valid and hook exists	OK (200)	The given Access Token replaces the stored one (if there was one in the Token Repository already)
Private	The given Access Token is valid but hook does not exist	NOT_FOUND (404)	Access Token is removed from the Token Repository (if any was stored)
Private	The given Access Token is invalid but there's a valid stored Access Token and hook exists	OK (200)
Private	The given Access Token is invalid but there's a valid stored Access Token and hook does not exist	NOT_FOUND (404)	Access Token is removed from the Token Repository
Internal		INTERNAL_SERVER_ERROR (500)
Any	If anything goes wrong	INTERNAL_SERVER_ERROR (500)

• POST /projects/:id/webhooks validates project's hook prior to a new hook is created using the same rules as in the grid above. Then hook is created only for cases marked with NOT_FOUND and the hook creation endpoint responds with CREATED (201). For cases marked in the grid with OK, the hook creation endpoint responds with OK, too.

• POST /webhooks/events - the endpoint works as before except for private repos events when the endpoint tries to use a stored Access Token. In fact, an attempt to fetch an Access Token is performed regardless of project visibility and if there's no token found, there's an assumption that the event is for a Public project.

closes #42

[rokroskar]

@jachro is this ready to be tested?

[jachro]

@rokroskar not yet. I found a bug yesterday and some noise in the logs. I addressed most of that yesterday but need to re-test today.

[lorenzo-cavazzi]

Working for both public and private repos.

Can also be tried it in the UI with these:

• UI for configuring KG integration renku-ui#374
• Connect graph services renku-gateway#91