Select which signal contacts/room to not use in the bridge

[wiredone]

Example Use Case: Moe would like to use mautrix/signald for most of their Signal communications, with the exception of a few contacts. Frank is one of Moes' Signal contacts and has expressed a wish to only use E2EE and as a result doesn't like mautrix/signal. Moe really loves how mautrix/signal is a step towards the matrix dream and is happy to use it for most of the contacts. However Moe doesn't enjoy the idea of having messages from Frank and Franks' contact details decrypted on the server. Moe would love a tool that would support dropping a contact from the bridge.