Extend user settings so that a user can store their notifications preferences like https://github.com/owncloud/enterprise/issues/6715

New settings:

• List of available events. Needs "name", "description" (translated) "avaibleInApp", "availableInMail" (bool) and "sendInApp", "sendInMail" (bool) fields (names tbd)
• Mail cycles replaces disable_email setting (can be "never", "instant", "weekly", "daily")