Host entity type / bundle may not be "media/video" hard-coded

I guess something like

  // Get the field definition.
  $field_definition = $this->getFieldDefinition();

  // Retrieve the entity type ID and bundle.
  $entity_type = $field_definition->getTargetEntityTypeId();
  $bundle = $field_definition->getTargetBundle();

might be needed?

I'm also unsure why this hard-coded implementation exists:

  /**
   * {@inheritdoc}
   */
  public static function getMediaType() {
    return 'video';
  }

but didn't check it deeper, yet. Please do that. Maybe because by FileMediaFormatterBase this is more than just a file formatter... mhm. Anyway, if a custom "myvideo" media type exists with a video field, it should also work, but hard-coding would break it, right?

Also, I am a bit unsure, when the Remote Formatter should be applicable:

  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    if ($field_definition->getTargetEntityTypeId() !== 'media') {
      return FALSE;
    }

    if (parent::isApplicable($field_definition)) {
      $media_type = $field_definition->getTargetBundle();

      if ($media_type) {
        $media_type = MediaType::load($media_type);
        return $media_type && $media_type->getSource() instanceof OEmbedInterface;
      }
    }
    return FALSE;
  }

Especially the "OEmbedInterface" part makes this very restricted to use.

@grevil re #6 LGTM so far... let's keep this for now.

Ok, please review then!

Nice!