This commit removes the restriction that was added in 4a61e456 to
prevent / from being used as the management context path when the
management context was not using a different port

The management context path can now be set to / irrespective of the
configuration of the management port. To avoid a possible clash
with the application's welcome page or similar, the links "endpoint"
that is mapping to the management context path is disabled when
the management context path is /.

As part of allowing / to be used as the management context path again,
the handling of endpoint mappings and the creation of paths for
individual operations has been consolidated into a new EndpointMapping
class that is used across the three (MVC, WebFlux, and Jersey)
implementations.

See gh-9898