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
A develop-only follow up sounds like the best plan to me, although I think system requirements may warrant a wider discussion - Mark has a point about some institutions being tided and we don't want to make these people think they can't run OMERO unless it is really true.