Sign add-ons with recommendation signer based on DiscoveryItem.

[EnTeQuAk]

This also prepares for the possibility that we may have to use different
credentials for the recommendations signer. In case we don't, ops only
has to set the creds twice which seems fair.

Fixes mozilla/addons#6522

[EnTeQuAk]

These credentials have to match https://github.com/mozilla-services/autograph/blob/master/autograph.yaml#L1053 (and respectively for alice)

[diox]

This makes sense. Do we have any end-to-end test in reviewer tools that include signing ? Maybe it's worth adding one w/ a recommendable add-on...

[EnTeQuAk]

This makes sense. Do we have any end-to-end test in reviewer tools that include signing ? Maybe it's worth adding one w/ a recommendable add-on...

We don't unfortunately,

addons-server/src/olympia/reviewers/tests/test_utils.py

Lines 1289 to 1315 in 0a1419c

	def test_nominated_to_approved_recommended(self):
	DiscoveryItem.objects.create(
	addon=self.addon, recommendable=True)
	assert not self.addon.is_recommended
	self.test_nomination_to_public()
	del self.addon.is_recommended
	assert self.addon.current_version.recommendation_approved is True
	assert self.addon.is_recommended
	def test_approved_update_recommended(self):
	DiscoveryItem.objects.create(
	addon=self.addon, recommendable=True)
	assert not self.addon.is_recommended
	self.test_public_addon_with_version_awaiting_review_to_public()
	del self.addon.is_recommended
	assert self.addon.current_version.recommendation_approved is True
	assert self.addon.is_recommended is True
	def test_autoapprove_fails_for_recommended(self):
	DiscoveryItem.objects.create(
	addon=self.addon, recommendable=True)
	assert not self.addon.is_recommended
	self.request.user = UserProfile.objects.get(id=settings.TASK_USER_ID)
	with self.assertRaises(AssertionError):
	self.test_nomination_to_public()
	assert self.addon.current_version.recommendation_approved is False
	assert not self.addon.is_recommended

tests this mostly but doesn't test the whole flow. Guess it's worth adding a test for that that doesn't rely on mocking.

[EnTeQuAk]

Maybe it's worth adding one w/ a recommendable add-on...

Added d99ab37 regarding end-to-end tests.