Add source zip to the source code section in the OMERO downloads page (rebased onto develop)



This is the same as gh-46 but rebased onto develop.

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With https://github.com/openmicroscopy/openmicroscopy/pull/2064, the OMERO source will be generated as a download artifact. This PR adds the zip file to the source code section of the OMERO downloads page.

/cc @rleigh, @gusferguson, @hflynn