Implement MSC3952: intentional mentions (#3092)
* Add experimental push rules.
* Update for changes to MSC3952: Use event_property_is and event_property_contains.
* Revert custom user/room mention conditions.
* Skip legacy rule processing if mentions exist.
* Add client option for intentional mentions.
* Fix tests.
* Test leagcy behavior with intentional mentions.
* Handle simple review comments.
