12252 script limit

See http://trac.openmicroscopy.org.uk/ome/ticket/12252

As a first step, make the script timeout configurable at the server-wide level (see `omero.scripts.timeout`; default 1 hour still). It may be that this is the limit of what we can do for 5.0.3, in which case, we should likely just test that it's possibly to reduce/increase the value in this PR, and save further advancements for 5.1.

/cc @emilroz @manics @jjcorreao
mention which units it's in