Feature: late registration

[wmurphyrd]

New feature to allow guests to join spaces, controlled by IMMERS_ALLOW_GUESTS environment var or meta tag

• Allow people to join as guest without creating an account
  • For the most part. this "just works" for the not-logged-in user because we already had Immers features gated on authorization scopes and they just don't have any scopes authorized
• For logged-in users interacting with guest users, update the hover menu (block-button, follow-button) based on whether the remote user is logged in (remove add friend, block becomes hide)
• Add "Claim account" prompts to allow registration later during session (this will prefill your current name in the reg. form and save your current avatar to your profile after it is created)
  • Desktop prompt is in bottom toolbar
  • VR prompt is in HUD (there seems to be a bug in Hubs's VR->2D->VR flow such that the room will be crashed when you come back, but it looks like this is fixed in the most recent Hubs Cloud release)
• Fix a bug in the popup auth flow that was preventing detection of when the popup closed

immers-space/immers#39 is not required for this to work, but it does work well with the late registration flow so that the display name doesn't have to be entered