Followup f1d5b69293
We want to make it super clear that this is to be removed when we merge
full width into core, keeping it in desktop.scss together with other
styles will make that harder.
Removes the [full-width
component](https://github.com/discourse/discourse-full-width-component)
dependency by adding most of the code directly to the theme. There was
one grid definition in the CSS I ended up removing because it was
conflicting with an existing override in the theme.
Anyone who has previously installed the theme should manually remove the
full-width component from it.
