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/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/notification/NotificationMapper.kt

@@ -53,6 +53,7 @@ class NotificationMapper(
                 timestamp = item.timestamp() ?: clock.epochMillis(),
                 content = item.event.use { notificationContentMapper.map(it) },
                 contentUrl = null,
+                hasMention = item.hasMention.orFalse(),
             )
         }
     }