Implement MSC2326 (Label based filtering)

[matrixbot]

This issue has been migrated from #6288.

---

Implement matrix-org/matrix-spec-proposals#2326 in Synapse

PRs:

• Dummy issue #6301 (basic support)
• Dummy issue #6310 (background job to support previous events)
• Dummy issue #6329 (support on /context, also on /search and some test refactoring)
• Dummy issue #6340 (fix SQL used to query events when paginating)
• Support edits (was Dummy issue #6374)