MSC4380 aims to be a simplified implementation of MSC4155; the hope is
that we can get it specced and rolled out rapidly, so that we can
resolve the fact that `matrix.org` has enabled MSC4155.
The implementation leans heavily on what's already there for MSC4155.
It has its own `experimental_features` flag. If both MSC4155 and MSC4380
are enabled, and a user has both configurations set, then we prioritise
the MSC4380 one.
Contributed wearing my 🎩 Spec Core Team hat.
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