Merge tag 'v1.27.0rc2' into develop

Synapse 1.27.0rc2 (2021-02-11)
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Features
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- Further improvements to the user experience of registration via single sign-on. ([\#9297](https://github.com/matrix-org/synapse/issues/9297))

Bugfixes
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- Fix ratelimiting introduced in v1.27.0rc1 for invites to respect the `ratelimit` flag on application services. ([\#9302](https://github.com/matrix-org/synapse/issues/9302))
- Do not automatically calculate `public_baseurl` since it can be wrong in some situations. Reverts behaviour introduced in v1.26.0. ([\#9313](https://github.com/matrix-org/synapse/issues/9313))

Improved Documentation
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- Clarify the sample configuration for changes made to the template loading code. ([\#9310](https://github.com/matrix-org/synapse/issues/9310))