Move creation of /tags facnav node from Essentials plugin to CMS plugin

the tags plugin creates, in a post-installation step (see TaggingInstructionPackage) the /tags node to set up faceted navigation for tags. This doesn't play well with the new Configuration Management mechanism, because the system then considers the node to be of category system, and it won't be registered in the YAML sources.

Since the /content node is now (CMS 12) contributed by the REPO and has a stable UUID (ed307dfc-5774-4ac7-9feb-79b57e058bb2), we can add the creation of the tags node to the YAML of the CMS plugin, and remove this post-installation step from Essentials.