Switch to third-party health API

[mschechter-bellese]

User Story

As an application maintainer
I'd like to use a third-party health framework
So that I can reduce maintenance overhead

Acceptance Criteria

• Demonstrate that the health API works as expected
• Demonstrate that if the database is off-line, the application is unhealthy

Notes

API choices:

• https://github.com/AMhaish/node-healthchecks-api
• https://github.com/hmcts/nodejs-healthcheck