OMERO.py downloads

There have been various requests for a Python bundle (e.g. https://www.openmicroscopy.org/community/viewtopic.php?f=6&t=7516&p=13982). Though this isn't yet pip-installable, having the download clearly marked on the downloads page is a start. **Note:** the download is available for previous versions as well, though it's likely not worth changing previous download pages:

```
 /ome/data_repo/public/omero $ ls -1 5.0.*/artifacts/OMERO.py*ice33*zip
5.0.0-rc1/artifacts/OMERO.py-5.0.0-rc1-ice33-b10.zip
5.0.0-rc2/artifacts/OMERO.py-5.0.0-rc2-ice33-b14.zip
5.0.0/artifacts/OMERO.py-5.0.0-ice33-b19.zip
5.0.1/artifacts/OMERO.py-5.0.1-ice33-b21.zip
5.0.2-rc1/artifacts/OMERO.py-5.0.2-rc1-ice33-b22.zip
```

/cc @sbesson @ximenesuk @hflynn @pwalczysko @manics @chris-allan