Omero downloads page

This PR uses the HTML template designed by @gusferguson to generate the OMERO downloads page. 

- the Pelican step is skipped for OMERO and no intermediate Markdown file is created
- an additional token is created for the artefact size and logic is added to doc_generator.py to compute human-readable file size
- the images/ folder is copied during `make omero`

/cc @hflynn