Dungeon - Session creation and deletion

[gwagwaflamingo]

A dungeon session keeps track of instances, dungeon data, and players assigned to the session.

A dungeon session can only be created if all limits are fulfilled by the party.

A session deletion refers to when a session is completely deleted, caused either by party disbanding or the reset dungeon button.

[AngeloTadeucci]

Session creation and deletion are fully implemented. Initial DungeonField infrastructure was added in PR #341 (commit 27b01981), with further adjustments in PR #375 (f82310fe) and PR #519 (495b0442). CreateDungeon() sets up lobby + sub-fields, and DestroyDungeon() cleans them up.