Andrew Shu authored Jul 29, 2020 and Deimos committed Aug 01, 2020

Adds the HTML Tidy library to the dev version, along with the pytidylib
wrapper for it, and a couple of tests that use it to validate the HTML
of the home page.

Includes a fix to the GitLab "Planned features" link that Tidy considers
invalid because it includes some un-encoded characters.