Merge tag 'v1.19.3' into release-v1.20.0

1.19.3

Synapse 1.19.3 (2020-09-18)
===========================

Bugfixes
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- Partially mitigate bug where newly joined servers couldn't get past
events in a room when there is a malformed event.
([\#8350](https://github.com/matrix-org/synapse/issues/8350))