Enhancement: Notifications

Make Emails translatable via transifex
The transifex translation add in to the email templates.
The optional environment variable NOTIFICATIONS_TRANSLATION_PATH added to config.
The optional global environment variable OCIS_TRANSLATION_PATH added to notifications and userlog config.

https://github.com/owncloud/ocis/pull/6038
https://github.com/owncloud/ocis/issues/6025
