Server upgrade bugfix (rebased onto dev_5_0)



This is the same as gh-604 but rebased onto dev_5_0.

----

This PR fixes a bug in the server upgrade page reported by @dpwrussell. As part of this fix, this PR addresses one of the suggestions in https://trac.openmicroscopy.org.uk/ome/ticket/10463, namely symlinking the new server. This will also greatly simplify subsequent upgrade steps with ``  $ cd OMERO.server``.