Send email invitation for user share

[skjnldsv]

For the user share, we should send an invitation, like calendar, mails, sharing... etc

[susnux]

Email notifications for user shares are already configurable using the notifications apps, so not sure if we really need a separate implementation for this, especially as users then would have to configure they do not want emails twice (in /settings/user/notifications and in /settings/user/forms). This would also require a per-user settings page.