Support MSC3952 intentional mentions

### Description

[MSC3952](https://github.com/matrix-org/matrix-spec-proposals/pull/3952) defines a structured way to mention other users (instead of parsing the plaintext `body`), see the MSC or [the spec](https://spec.matrix.org/v1.7/client-server-api/#user-and-room-mentions) for more information. This has been accepted and is included in Matrix 1.7.

Support for this is currently being rolled out in Synapse and Element Web (see vector-im/element-meta#886), unfortunately without mobile application support it is likely that users will not fully benefit from the changes:

* Users may still receive unintentional pings on their mobile devices.
* Messages sent from mobile devices may still induce unintentional pings on others, regardless of platform.

Essentially mobile devices will exhibit today's legacy behavior and continue to propagate buggy mentions.