Decide whether we want to support sending membership events for users that do not exist

This issue has been migrated from [#7937](https://github.com/matrix-org/synapse/issues/7937).

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### Description

It is currently possible to send membership events (invite, kick, etc.) about users that are not known to the sending server. Typically this has not been a problem, but can arise in confusing behaviour such as when a user registers a new account and finding that they have existing invites (https://github.com/matrix-org/synapse/issues/7898).

It can also result in oddities such as being able to kick users from a room that do not exist (https://github.com/matrix-org/synapse/issues/2959).

However it can be advantageous to ban a user from a room even if the homeserver they're on claims they do not exist, therefore we may need to think about this on a per-membership event type basis.