Stabilize support for MSC2675: /relations endpoint

This issue has been migrated from [#11753](https://github.com/matrix-org/synapse/issues/11753).

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[MSC2675](https://github.com/matrix-org/matrix-doc/pull/2675) is currently in FCP (to complete tomorrow, barring any concerns). We have a few bits of work to do to stabilize support in Synapse for it:

* [x] Fix-up redaction support:
* Ensure that redacted relations are not bundled nor returned from `/relations`.
* Support fetching relations of a redacted event. (Currently this returns nothing.)
* Bundle aggregations for a redacted event.
* [x] Properly handle relations for ignored users in:
* [x] `/relations`
* [x] Bundled aggregations.
* [x] Support accepting pagination tokens from `/sync` and `/messages`.
* [x] Clean-up backwards compatibility with the old pagination token format.
* [x] Add the `v1` prefix for `/relations`.
* [ ] Remove the `unstable` prefix for `/relations`.

Those were all the differences that I found by reading through it, but there could be more.