Registration tokens

[Twi1ightSparkle]

Is your feature request related to a problem? Please describe.

Registration tokens (https://element-hq.github.io/synapse/latest/usage/administration/admin_api/registration_tokens.html#registration-tokens) in Synapse are used by both our customers and community servers to mitigate abusive account registrations.

Describe the solution you'd like

Implement registration tokens in MAS.

Describe alternatives you've considered

Other than not using MAS and by extension not use Element X, no good alternatives are available. Email allow list is not a practical replacement for community instances.

[kodxana]

+1 it would help onboard more people without to having to create new user each time manually.

[kodxana]

@sandhose Would it be hard to change registration flow?
Curently user gets whole form to fill then at end it's asked to provide token.
Having token first then registration form would be less confusing.