Be able to restrict which matrix users can register

This solves #5 (closed). I have a few questions:

  1. What should the default be? Currently, it's all users, but maybe it can be all users on the current homeserver
  2. What should the default for dev be? It's that dev1 and dev2 can register but dev3 cannot atm
  3. I have no idea how testing would be added for this

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