Bio-Formats downloads page

The Bio-Formats downloads page was originally moved to https://github.com/openmicroscopy/ome-release. As part the ongoing effort to simplify this release process and have simple mock releases (https://trac.openmicroscopy.org.uk/ome/ticket/11354), the main proposal of this PR is to keep/maintain the downloads pages as part of the code repository itself.

Additionally the downloads page has been sphinx-ified so that:
- it could be built and archived together with the job artifacts and very easily deployed directly to http://download.openmicroscopy.org/. Alternately, the downloads page could be built at release time only.
- modifications of the downloads page can be easily proposed by PR
- the HTML theme can be easily unified with the one used by traditional documentation/products pages using the sphinx_theme.git repository

/cc @hflynn, @joshmoore