Add a status report that checks if all adapters are running

We should implement a check that could appear on the status report page that checks if all the WissKI Salz Adapters are configured correctly.

In particular, this could run the following checks:

• There should be active Preferred Local Store
• Each adapter has configuration settings, i.e. a valid read URL and a valid write URL (the latter only if PLS)
• Each adapter is reachable, i.e. it does not produce an error when being queried with something like SELECT (count(*) as ?c) WHERE { ?s ?p ?o } (except if it doesn't do raw queries)

AFAIK status check should be implemented by implementing hook_requirements with $phase == 'runtime'.