[SPARK-19220][UI] Make redirection to HTTPS apply to all URIs.

The redirect handler was installed only for the root of the server;
any other context ended up being served directly through the HTTP
port. Since every sub page (e.g. application UIs in the history
server) is a separate servlet context, this meant that everything
but the root was accessible via HTTP still.

The change adds separate names to each connector, and binds contexts
to specific connectors so that content is only served through the
HTTPS connector when it's enabled. In that case, the only thing that
binds to the HTTP connector is the redirect handler.

Tested with new unit tests and by checking a live history server.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #16582 from vanzin/SPARK-19220.