Merge tag 'v1.7.2'

Synapse 1.7.2 (2019-12-20)
==========================

This release fixes some regressions introduced in Synapse 1.7.0 and 1.7.1.

Bugfixes
--------

- Fix a regression introduced in Synapse 1.7.1 which caused errors when attempting to backfill rooms over federation. ([\#6576](https://github.com/matrix-org/synapse/issues/6576))
- Fix a bug introduced in Synapse 1.7.0 which caused an error on startup when upgrading from versions before 1.3.0. ([\#6578](https://github.com/matrix-org/synapse/issues/6578))