Merge tag 'v1.20.0rc5' into develop

Synapse 1.20.0rc5 (2020-09-18)
==============================

In addition to the below, Synapse 1.20.0rc5 also includes the bug fix that was included in 1.19.3.

Features
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- Add flags to the `/versions` endpoint for whether new rooms default to using E2EE. ([\#8343](https://github.com/matrix-org/synapse/issues/8343))

Bugfixes
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- Fix rate limiting of federation `/send` requests. ([\#8342](https://github.com/matrix-org/synapse/issues/8342))
- Fix a longstanding bug where back pagination over federation could get stuck if it failed to handle a received event. ([\#8349](https://github.com/matrix-org/synapse/issues/8349))

Internal Changes
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- Blacklist [MSC2753](https://github.com/matrix-org/matrix-doc/pull/2753) SyTests until it is implemented. ([\#8285](https://github.com/matrix-org/synapse/issues/8285))