Add plain text representation of messages (#1850)

* Add plain text representation of messages.

This is used in the room list as the last message in a room, in the message summary when a message is selected, in the 'replying to' block, in the 'replied to' block in a message in the timeline, and in notifications.

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/notification/NotificationData.kt

@@ -36,6 +36,7 @@ data class NotificationData(
     val content: NotificationContent,
     // For images for instance
     val contentUrl: String?,
+    val hasMention: Boolean,
 )
 
 sealed interface NotificationContent {