Expose /healthcheck endpoint for publisher
Expose /healthcheck endpoint for publisher

The original reasoning for hiding this endpoint [1] has been superseded
by many other publishing apps, which expose this endpoint [2][3].

[1]: https://github.com/alphagov/govuk-puppet/commit/7b18de8a9a4a046798ee8204a38a762dac395fd4
[2]: https://github.com/alphagov/govuk-puppet/commit/ffe22090729a8a1ea72d580925b2e4695e8ff3c0#diff-ebf0b64802b614154eb3e90a9a8f5ff4R61
[3]: https://github.com/alphagov/govuk-puppet/commit/d37875f12882666918ae436adb126dfbcd69ef50#diff-0311ac08466d9b3be654cc68f14474e8R79

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