Add Pathauto support for group content entities

Follow-up from #2693589: Refactor the way GroupContent exposes link relations and templates

Basically, we need a [group-content:pretty-path] token or something along those lines so people can use Pathauto to generate pretty paths such as group/1/members/127 instead of group/1/content/127.

We also need to document this better:

  /**
   * (optional) The key to use in automatically generated paths.
   *
   * Will be added to the entity tokens so modules like Pathauto may use it.
   *
   * @var string
   */
  public $pretty_path_key;