Convert theme to Tutor plugin

Get rid of the tutor config render command, which is useless now that themes can be implemented as plugins. (by @regisb)

-- Tutor changelog

The tutor config render command is currently used by the make command to compile the theme, so we can't recompile with any version of Tutor over 13.3.0

If we want to be able to keep using this theme in Tutor >13.3 (which I think we do), we need to turn it into a plugin. I think merging upstream into this repo is 90% of the work for achieving that.