Using right redirects for 5 downloads

Updating redirects we use for the downloads page which were agreed earlier today. With apologies to @joshmoore who tried to tell me before that /latest-develop/ was going to be thoroughly confusing!

To test, check that all the downloads links on https://www.openmicroscopy.org/site/support/omero5-staging/ go to the 5-rc1 downloads pages. These redirects will be updated to point at 5.0.0 when we release.

cc/ @sbesson @qidane