Scope the opened action of webhook to auth'ed issues

[karlcow]

This has to be done before #3140 and #3155

Currently the webhook will listen on any opened actions. It means that if we send POST requests coming from other repos. We will make the webhook busy for nothing. We need to scope it by checking the request is coming from the priviledged repo.

webcompat.com/webcompat/webhooks/__init__.py

Lines 40 to 42 in 236ac81

	if issue['action'] == 'opened':
	# we are setting labels on each new open issues
	response = new_opened_issue(payload)

In this way the webhook will not process anything in the case of opened actions on private issues.

[karlcow]

Here an example of a payload that the webhook receives (removing unnecessary details.)

{
  "action": "opened",
  "issue": {
    "url": "https://api.github.com/repos/webcompat/web-bugs/issues/47772",
    "repository_url": "https://api.github.com/repos/webcompat/web-bugs",
    "number": 47772,
    "title": "fireflix.ws - site is not usable",
    "user": {
…
    },
…
}