Move CMS components to Essentials blog plugin

The Essentials CMS Components artifact solely contains classes relevant in the context of Essentials' blog plugin. This is confirmed by the artifact being installed as a dependency of the project only when one installs the blog plugin. As such, these classes should live in the same (git) project as the blog plugin itself, in order to improve maintainability and clarity.

I suggest to move and rename the artifact in the context of the 13.0 release. Care should be taken not to lose history on the code. And this represents a backwards incompatible change (if the artifact name is changed), so it will need to be documented in the upgrade steps.