Make members list prefix configurable throug the UI

Currently the text 'You and' is fixed in PrivateMessageThreadMemberFormatter.php in the function viewElements:

'#markup' => '<span>' . $this->t('You and') . ' </span>' . implode($separator, $users),

It would me much nicer to remove it or to include it as setting to the form. Especially when the display mode is set to entity this fixed text isn't usefull.