Add verifiability for Slack webhooks

[KevinHock]

As @dgzlopes helpfully linked to, https://github.com/streaak/keyhacks#slack-webhook shows how we can add this.

See #194 for an example of how we do this in detect-secrets, e.g. we already have validation for Slack tokens

detect-secrets/detect_secrets/plugins/slack.py

Lines 30 to 39 in cba0446

	def verify(self, token, **kwargs): # pragma: no cover
	response = requests.post(
	'https://slack.com/api/auth.test',
	data={
	'token': token,
	},
	).json()
	return VerifiedResult.VERIFIED_TRUE if response['ok'] \
	else VerifiedResult.VERIFIED_FALSE

The parent issue (#153) is still opened but figured I'd make this as a good first issue.

[Patil2099]

@KevinHock Can I work on this?

[KevinHock]

Sure thing @Patil2099 :)

Let me know if you have any questions.

[Patil2099]

@KevinHock Can you help me a little bit? I might be able to do it for mailchimp and stripe also.
Thanks!

[KevinHock]

Sure @Patil2099 , for this you would change the verify function of slack.py to something like

    def verify(self, token, **kwargs):  # pragma: no cover
        if token.startswith('https://hooks.slack.com/services/T'):
            # From https://github.com/streaak/keyhacks#slack-webhook
            response = requests.post(
                token,
                json={
                    'text': '',
                },
            )
            valid = response.text == "missing_text_or_fallback_or_attachments"
        else:
            response = requests.post(
                'https://slack.com/api/auth.test',
                data={
                    'token': token,
                },
            ).json()
            valid = response['ok']

        return (
            VerifiedResult.VERIFIED_TRUE
            if valid
            else VerifiedResult.VERIFIED_FALSE
        )

where the missing_text_or_fallback_or_attachments part is from the advice given here https://github.com/streaak/keyhacks#slack-webhook.

Afterwards you can make sure the tests still pass with make test (or Travis will run tests on your PR anyways).