Change Omeka database table prefix

This moves it inline with existing installations that have to be
migrated.  I assume that, at this early stage, nobody's relying on
the Omeka site yet in their development VMs. :-)
Fix directory obliteration in Exhibitions deployment

Exclude the correct "files" directory fromt the rsync command that
deploys the Exhibitions app, to keep its contents from being
deleted.  Also be more specific about the directory ownerships and
permissions in the "files" directory.