Merge tag 'v1.7.0rc2' into develop

Synapse 1.7.0rc2 (2019-12-11)
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Bugfixes
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- Fix incorrect error message for invalid requests when setting user's avatar URL. ([\#6497](https://github.com/matrix-org/synapse/issues/6497))
- Fix support for SQLite 3.7. ([\#6499](https://github.com/matrix-org/synapse/issues/6499))
- Fix regression where sending email push would not work when using a pusher worker. ([\#6507](https://github.com/matrix-org/synapse/issues/6507), [\#6509](https://github.com/matrix-org/synapse/issues/6509))