Granularize user preferences with regards to notifications

Vorto notifies users who have set a valid e-mail address of a number of events concerning them, ranging as changes in their roles in a namespace to comments on models they authored, etc.

It would be a good idea on the long run to allow users to decide which notifications they want to receive.

For now, this could just be a list of check boxes on the account settings page.

If we ever implement an alternate notification system that does not use e-mail (e.g. within the Vorto app), then we also add a toggle for each event type, to choose the notification mode.