This commit provides a single endpoint, /actuator, that serves HTML
(the HAL browser) or JSON depending on the request’s accept header
that enables discovery of all of the actuator’s other endpoints.

When the management context path is configured, the /actuator endpoint
moves to the configured path, e.g. if the management context path is
set to /management, the actuator endpoint will be available from
/management.

Closes gh-3696