Resolve "Websockets do not respect shared lab staff"

What does this mr do?

Websockets do not respect shared lab staff. Read issue for more desc (#555 (closed))

Actions taken to fix bug

WebsocketService no longer sends messages to everyone in the shared lab, but rather the staff of the edition that the assignment corresponds to.

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
• A test was created to test the bug.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #555 (closed)