Add Trusted-Only:true to the Insight manifest.

See http://docs.oracle.com/javase/6/docs/technotes/guides/jweb/mixed_code.html#trusted_only
Don't sign the webstart jars by default.

We're going to be code-signing in a separate process, so this prevents jars being double-signed, once with the self-signed cert and once with a proper one.

The webstart-sign target is still there, so you can use `./build.py webstart-sign release-webstart`.
Add an external webstart signing script

Automatically run this script with the existing self-signed certificate as part of the standard docs/hudson/OMERO.sh build
Merge pull request #2193 from manics/more_webstart_codesigning_tweaks

More webstart code-signing changes (merging for `5.0.1`)