Improve lobby creation to ensure that the host is automatically joined to the lobby

[jiyeongchloe]

Goals: Currently, we have functionalities for users to create lobbies. However, once they create a lobby, the lobby is displayed as empty. Intuitively, it's the most reasonable for the host of the lobby to already have joined the lobby at the time of creation rather than asking them to join their own lobby again. We want to improve the LobbyCreateView in views.py to ensure that the host is automatically added as a player in the lobby. In conjunction with Issue #406, we also need to make sure that the host is not currently in any other lobbies.

Deliverables:

• Pull Request