Werk #11917: Fix and rework user notification option "Send hint to ..."

The option "Send hint to message inbox (bottom of sidebar)" on the "Notify users" page was non functional in previous 2.0 versions.

This option has now been renamed to "Send hint to user menu". If a user notification is triggered with this option, the affected user will get a hint in the "User" entry of the main menu. In the user popup menu, a link to the user notifications is shown, including the count of new messages. If the user clicks on that link, he will get to a new page "User notifications", where he can read and delete new messages.

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