✨(jitsi) allow adding a prefix to the Jitsi room names

[sampaccoud]

Purpose

We may want to namespace room names in the same Jitsi instance or we may want to include a text in the room names.

Proposal

• Add a prefix to the Jitsi room name for both the Meeting and Room models
• Allow setting the Jitsi room prefix by setting the MAGNIFY_JITSI_ROOM_PREFIX environment variable
• Update the environment variables documentation to add the new setting