Composer for the win!

There are several ways that themes and plugins can be installed and activated.  Until now, we really had no defined path and some stuff was installed via composer, another was via wp-cli, and yet other stuff was either included (custom-theme) or complete ignored (wp-content + admin UI).  With these changes the following is happening:

* Installations are done with composer (themes and plugins are requirements after all)
* Activations are done with wp-cli (it's the API wrapper after all, and there is no reliable way to do this elsewhere)

With this, we have fewer things to worry about:

* WP_THEME setting is gone from .env.example .  If you need a different theme, simply update your composer.json and you'll have it.  Your custom child theme will need a template reference update as well.
* Installing themes with composer provides additional benefit of destination folder control.  No more do we need to worry about the theme version in the child theme (like wp-cli was installing it)
* Running wp-cli.install is not necessary anymore to just get all the components.  You'll still need it to activate stuff, but this will be handled better and cleaner in the later changes.