Revert to ec27f001718
Make RTD using our theme styling.
If html_theme is set, then RTD should respect that choice.
Redefine \H to be a shortcut for entropy.
Force theme.js and theme.css, even the READTHEDOCS env var is set.

This is an ugly temporary hack in use until we can customize the
new RTD theme from conf.py.  Then, we can just use the default
theme.