Mentions: Send intentional mentions

[Velin92]

From element-x-ios created by Velin92: element-hq/element-x-ios#1914

Use the new Rust SDK API RoomMessageEventContentWithoutRelation::with_mentions implemented here to include intentional mentions when sending or editing a message.

This requires the client to actually parse from the RTE and then add them before sending.
This requires the following RTE API to be implemented: matrix-org/matrix-rich-text-editor#848

[jmartinesp]

Depends on #1453 .