Transfer invites/rooms/etc. when a guest account is upgraded

This issue has been migrated from [#7918](https://github.com/matrix-org/synapse/issues/7918).

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**Description:**

Right now, when I want to PM a guest account as a registered user, I can initiate the conversation, but the guest can't accept the invitation, getting prompted to create an account instead. But after creating an account, the invite is no longer valid, since the name changed.

Also, the other way around, when I want to PM another user as a guest, it prompts me to create an account.

I think it would make sense to allow guests to participate in Private Message Rooms. If there are spam concerns, it could be realized by simply allowing guest accounts to accept invites to PM rooms by registered users