[Authenticated] Web client multiple sandboxes, teleportation to anywhere on server

[satoshinm]

Instead of only having one sandbox location (center_x/y/z+radius), allow it to be changed dynamically on-demand to anywhere on the server, for a given web client. Something like /websandbox tpahere user. Could be useful especially in conjunction with or similar to #47 spectator mode.

Requested in https://www.spigotmc.org/conversations/please-help-me-with-websandboxmc.211573/page-2#message-1263964

[satoshinm]

As part of this, also may want to start the authenticated (#48) player at the location where they last logged out. Save it as their last login location, possibly with a home location.

[satoshinm]

This would not require as invasive changes as multi-chunk support, since each client would still only see one chunk at (0,0) but it would be at a different location on the web client.