add activity notification mail template if activity app is enabled

@karlitschek @nickvergessen fix https://github.com/owncloud/activity/issues/80 by adding the notification template to the editable templates when the activity app is enabled.
Either the templateeditor app checks if the activity app is enabled and adds the notification template, or we add an api call that all other apps can user to edit php files directly. For now we don't want to openthat can of worms, so I decided to hard code it in the templateeditor app. We can move to an api when we are using sth. like twig that solves a lot of security issues arising with this.