improve and update server configuration documentation (rebased from develop)

Makes a start on the work for https://trello.com/c/Gpwp1WkC/188-improve-server-config-docs and removes some information about both OMEIS archeology and, from https://trello.com/c/avvIvqCk/67-5-0-3-follow-up, our long-neglected OpenGL clients.
--rebased-from #948
It's nonetheless very common for people to deploy software on known insecure and unsupported systems -- sometimes institutionally imposed! -- and that lack of support won't stop OMERO working just fine on them. I agree that your suggestion has significant merit, but I would much prefer to defer it to a meeting about prerequisite version numbers that we're going to have to have anyway, and deal with it as an outcome of that, thus keeping those worms largely in the can for this rebase PR which is basically about taking the easier steps of addressing comments from https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=7591.