Stabilize support for MSC2675: /relations endpoint

[matrixbot]

This issue has been migrated from #11753.

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MSC2675 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:

• 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.
• Properly handle relations for ignored users in:
  • /relations
  • Bundled aggregations.
• Support accepting pagination tokens from /sync and /messages.
  • Clean-up backwards compatibility with the old pagination token format.
• 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.