Reworked the party system + improvements

Support for going offline after closing the game, crashing or alt-f4. Updates all the members in the party
Reconnecting into the same party
Support for a offline party member
Bugfix switching to new party when changing characters
Bugfix staying in a old party after party does not exist anymore (after closing and reopening the game)
Bugfix memory leak in the party system
Bugfix using expensive search (in all the sessions) when we have the party in the session

[ ] This change introduces new dependencies and I have included updated pyproject.toml and poetry.lock files.
[ ] This change requires database migrations and I have included an alembic migration.

Snaacky / dndserver