Option to Shadow Ban users

[JokerGermany]

Is your feature request related to a problem? Please describe.
Deactivating a user should be the last resort. If you want to stop a user and check the situation, deactivate is a little bit to much.
Even if i want to let another one control my decision but need to act now, a shadow ban would be helpful

Describe the solution you'd like
!mjolnir shadow-ban <user IDs>
Would be great if you could add several user IDs to the command.
https://matrix-org.github.io/synapse/latest/admin_api/user_admin_api.html#controlling-whether-a-user-is-shadow-banned

Describe alternatives you've considered
none

[heftig]

Note that you can only shadow-ban users belonging to your own homeserver, as it affects the client API, not federation.

[turt2live]

Duplicate of #82