* enable feature_intentional_mentions with v1.7
* org.matrix.msc3952.mentions -> m.mentions
* update push rules test util
* only support stable version 1.7
* use stable values for msc3952 push rules
* use stable intentional mentions rule sin test models
* unstable feature in settings controller
Implements the intentional mentions feature of MSC3952 (behind
a labs flag).
If enabled, this will send an org.matrix.msc3952.mentions property
on events that will contain the user IDs and/or whether the room is
being mentioned. These mentions also gets propagated via some
custom behaviour for replies and edits.
