Use shared redis for OMERO.web sessions

[manics]

Each OMERO.web instance was given its own redis server whilst investigating a major problem with sessions on the IDR: #38

This should be reverted to support future microservices which require a common session store.

[manics]

I don't think a single shared redis database is possible at the moment, because OMERO.web sessions are tied to a single OMERO.server. I think multiple OMERO.servers would need to store their sessions in the shared redis first.