BUG: Site tree sidebar not updated when creating a new translation
BUG: Site tree sidebar not updated when creating a new translation

The site tree sidebar was not updated when creating a new translation,
due to only CurrentForm and Breadcrumbs fragments being updated by default.

Fixed by explicitly setting the X-Pjax response header to Content on
translation creation.

tractorcow/silverstripe-translatable,JayDevlin/silverstripe-translatable,JayDevlin/silverstripe-translatable,dnadesign/silverstripe-translatable,CrackerjackDigital/silverstripe-translatable,dnadesign/silverstripe-translatable,tractorcow/silverstripe-translatable,tractorcow/silverstripe-translatable,CrackerjackDigital/silverstripe-translatable