Plugins: Disable uninstallation when group content exists for it

GroupContentEnabler plugins are responsible for the automatic creation of GroupContentType entities. When a plugin is disabled on a GroupType, the GroupContent that had the associated GroupContentType as its bundle will become orphaned. This is bad :)

Tasks:

• Disallow the uninstallation of a plugin from a GroupType when there is GroupContent using its GroupContentType
• Disallow the uninstallation of modules providing plugins matching the above criteria

Tip: Create a service for the second task.

  group.uninstall_validator:
    class: Drupal\group\Extension\GroupContentUninstallValidator
    arguments: ['@plugin.manager.group_content_enabler', '@entity_type.manager', '@string_translation']
    lazy: true
    tags:
      - { name: module_install.uninstall_validator }