[enhancement]: Manage untrusted sieve from web client

[Megver83]

Which feature or improvement would you like to request?

I'd like to see this feature: just like admins can manage global and trusted sieve scripts from the web client, users could do the same from the self-service portal to manage their own ("untrusted") scripts. An interface like Roundcube's Managesieve plugin would make it easier for all users, since it's user-friendly and also has the option to directly write the scripts.

Is your feature request related to a problem?

Managing sieve in a user-friendly way it's a PITA, e.g. Roundcube's solution is pretty good but I don't want to install a whole webmail just for this. There are more (third-party) alternatives, but if Stalwart could include one untrusted scripts, that would be awesome.

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