Add a function to put a mail on hold until en configurable date
As a user, after reading a mail, I would like to be able to put it on hold until a certain date (e.g. event date). The callback date can be configured by the user.
When the mail is on hold it is no longer visible in the inbox. However, it can be searched to be read.
At the time of the reminder, the mail is announced again in the mailbox.