Add a version footer

[bess]

• I should be able to tell what version of the software is deployed on a server by looking at the page footer

See, e.g., the footer at https://nurax.curationexperts.com/

Breadcrumbs:
https://github.com/curationexperts/nurax/blob/master/app/views/shared/_footer.html.erb
https://curationexperts.github.io/playbook/every_project/git_sha.html