MSC3952 defines push rules which searches for mentions in a list of
Matrix IDs in the event body, instead of searching the entire event
body for display name / local part.
This is implemented behind an experimental configuration flag and
does not yet implement the backwards compatibility pieces of the MSC.
* Support MSC1767's `content.body` behaviour in push rules
* Add the base rules from MSC3933
* Changelog entry
* Flip condition around for finding `m.markup`
* Remove forgotten import
* Add support for MSC3931: Room Version Supports push rule condition
* Create experimental flag for future work, and use it to gate MSC3931
* Changelog entry
