Bio-Formats downloads page (rebased onto dev_4_4)

This is the same as gh-681 but rebased onto dev_4_4.

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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