Merge tag 'v1.21.0rc2' into develop

Synapse 1.21.0rc2 (2020-10-02)
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Features
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- Convert additional templates from inline HTML to Jinja2 templates. ([\#8444](https://github.com/matrix-org/synapse/issues/8444))

Bugfixes
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- Fix a regression in v1.21.0rc1 which broke thumbnails of remote media. ([\#8438](https://github.com/matrix-org/synapse/issues/8438))
- Do not expose the experimental `uk.half-shot.msc2778.login.application_service` flow in the login API, which caused a compatibility problem with Element iOS. ([\#8440](https://github.com/matrix-org/synapse/issues/8440))
- Fix malformed log line in new federation "catch up" logic. ([\#8442](https://github.com/matrix-org/synapse/issues/8442))
- Fix DB query on startup for negative streams which caused long start up times. Introduced in [\#8374](https://github.com/matrix-org/synapse/issues/8374). ([\#8447](https://github.com/matrix-org/synapse/issues/8447))