Port "Allow users to click account renewal links multiple times without...

Port "Allow users to click account renewal links multiple times without hitting an 'Invalid Token' page #74" from synapse-dinsic (#9832)

This attempts to be a direct port of https://github.com/matrix-org/synapse-dinsic/pull/74 to mainline. There was some fiddling required to deal with the changes that have been made to mainline since (mainly dealing with the split of `RegistrationWorkerStore` from `RegistrationStore`, and the changes made to `self.make_request` in test code).