Add support for UserList

[shiqimei]

Technical details:

1. display a UserList in the right to replace the current buttton (refers to our UI design);
2. to display all online users on this board (refer to js.design)
3. we don't need to support watch in the MVP version
4. ExcalidrawSyncService, Collab.tsx
   • room - boardId
   • getUserListByBoardId (not from MongoDB, from SyncService memory)
5. no permission control in the MVP version

Behaviour:

• A, B on the board http://localhost:3000/board/yidoD7FxvDNwNsRjh, UserList should be A, B.
• A, B, C -> A, B +1

[shiqimei]

1. auto join room (sync service)
2. provide interface, public getUserListByBoardId

[raazseth]

Can you assign me this issue @shiqimei @ccbond

[CulturalProfessor]

@shiqimei can i start on this issue I think it will be helpful in project also ?