Add endpoint to lookup a notification by external key

[briehl]

Works like this:
GET /api/V1/notification/external_key/<key>

This only works when the auth header contains a Service token, and that service was used to create the Notification. That is, the source key matches the service.

If those criteria aren't matched, returns a 404 error.

[coveralls]

Pull Request Test Coverage Report for Build 145

• 17 of 17 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.849%

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Totals
Change from base Build 144:	0.0%
Covered Lines:	963
Relevant Lines:	1060

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💛 - Coveralls