probe: check availability of UI

[hjhsalo]

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.