Add the health_check gem end expose the health_check route

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/7887

This adds a health_check controller on /health_check (.html, .json ,.xml) and health_check/checktype (.html, .json, .xml) for example health_check/cache.json By default it checks the database, and cache.

The endpoint is only accessible if you pass a valid health check access token. A new page in the admin section titled Heath Check has been added in order to show the current token and allow you to reset it.