Implement Portal Migration

[Zintixx]

Need to add functionality for using a portal to migrate to a different channel and specified position/rotation.
This is needed for portals such as Guild/Party summoning portals.

[AngeloTadeucci]

Implemented in PR #533 (commit 5b81b24b) and fixed edge cases in PR #539 (commit 4c39caf7). Field-to-field and field-to-home portal migration are now fully handled.