Mobile multiplayer (and minor fixes)

I found out while testing the game with some friends that there were several issues with the itch.io deployment, whereas these problems did not exist on https://maginet.evrim.zone/.

The issue was that if you created an online lobby on itch.io, on a mobile device, the embedded iframe security policies did not allow the session cookie to be set for the game. This meant that the game would not discover your session ID and report it to the server, and in turn you wouldn't actually join the lobby.

In this version:

1. Sessions are now stored with localStorage (https://developer.mozilla.org/en-US/docs/Web/API/Web_Storage_API) and no longer with cookies. Further steps are needed to obfuscate and secure the session IDs, but currently these should not be necessary.
2. Right-clicking with a mouse deselects the active mage.
3. Several other fixes relating to content caching (server-side) which should make updates be delivered more reliably.

With this update, mobile lobbies should now be playable inside itch.io!