Add consent tracking

[S7evinK]

This adds the unspecified consent tracking. Most parts are documented here

• When creating a new account, the user now has to accept the specified terms an services (or what ever you call it via the config)
• If enabled after the account was created, the user is blocked from sending any messages/joining rooms and will receive a message to accept the terms before being able to do anything
• Asking the user to accept the policy (e.g. via server notices) can be done with #2180. (once merged, needs additional work)