Deprecate and move the Content plugin into a legacy sub-module

Problem/Motivation

Providing a Content plugin for creating content entities managed via Parameters was a huge mistake. It needs to be moved out of the main module into a sub-module so that existing sites making use of this plugin keep working.

Steps to reproduce

Proposed resolution

Remaining tasks

• Move everything related to the Content plugin into a new sub-module
• Mark the plugin as deprecated

User interface changes

API changes

Data model changes