Update webadmin_utils.py

http://trac.openmicroscopy.org.uk/ome/wiki/MilestoneDownloads has a big red sign stating "deprecated page", with a link to donwloads.o.org. Might as well direct users to downloads.o.org from the application in the first place.
Update webadmin_utils.py

Changed to http://downloads.openmicroscopy.org/latest/omero/ as per Andrew's suggestion.
Merge pull request #3004 from kennethgillen/patch-1

Update webadmin_utils.py with downloads.o.org link instead of trac.o.org