probe: check availability of UI

Maybe just simple HTTP GET and check that the fetched HTML file looks as it should look.
On the other hand, changes to the fetched index page (done in https://github.com/EUDAT-B2SHARE/b2share) might need to be propagated to monitoring probe.
This obviously causes more administrative work, so benefits of checking HTML vs checking for simple 200 OK, should be weighted with care.