Support MSC3952 intentional mentions

[clokep]

MSC3952 defines a structured way to mention other users (instead of parsing the plaintext body), see the MSC or the spec 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.

[clokep]

At the very least it should likely be confirmed that unknown push conditions (and rules) are properly ignored and will not break anything.