Merge tag 'v1.2.1' into shhs shhs-v1.2.1

Synapse 1.2.1 (2019-07-26)
==========================

Security update
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This release includes *four* security fixes:

- Prevent an attack where a federated server could send redactions for arbitrary events in v1 and v2 rooms. ([\#5767](https://github.com/matrix-org/synapse/issues/5767))
- Prevent a denial-of-service attack where cycles of redaction events would make Synapse spin infinitely. Thanks to `@lrizika:matrix.org` for identifying and responsibly disclosing this issue. ([0f2ecb961](https://github.com/matrix-org/synapse/commit/0f2ecb961))
- Prevent an attack where users could be joined or parted from public rooms without their consent. Thanks to @Dylanger for identifying and responsibly disclosing this issue. ([\#5744](https://github.com/matrix-org/synapse/issues/5744))
- Fix a vulnerability where a federated server could spoof read-receipts from
  users on other servers. Thanks to @Dylanger for identifying this issue too. ([\#5743](https://github.com/matrix-org/synapse/issues/5743))

Additionally, the following fix was in Synapse **1.2.0**, but was not correctly
identified during the original release:

- It was possible for a room moderator to send a redaction for an `m.room.create` event, which would downgrade the room to version 1. Thanks to `/dev/ponies` for identifying and responsibly disclosing this issue! ([\#5701](https://github.com/matrix-org/synapse/issues/5701))

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