managesieve vacation: restrict :from to identities (on identies_level=1/3)

[RolandRosenfeld]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have searched for plugins that provide already provide the feature

Proposal

On systems with identities_level 1 or 3 (user isn't allowed to enter a random mail address), it would be a good idea to to change the "Reply sender address" entry in managesieve vacation from a free text field to a pulldown menu with default empty string and a list of mail addresses from identities as other alternatives.

Motivation and context

Otherwise user may be able to enter some wrong mail address.
I know, that using the "Edit filter set" option, an experienced user can enter everything, but this would reduce typos and mistakes by basic users.