Proposed changes to wd_s

- Move project specific mu-plugin files into the _s folder for better organization
- Replaced _s with submodule of wds/_s
- Created sample plugin and start registering cpts/taxonomies in "feature plugins" instead of in mu-plugins
- Disable easy deactivation of plugins, remove deactivate/edit links and replace with "WDS Required Plugin"

As far as I can tell we cannot easily short circuit the deactivate_plugin functino