Send read receipts (#713)

* Send read receipts

* Add changelog

* Add tests.

* Optimise how sending read receipts work in the timeline.

* Move the check for send read receipts to the presenter

Also improve the way we find the first visible `Event` if we have `Virtual` ones in the timeline.

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/timeline/MatrixTimeline.kt

@@ -32,4 +32,6 @@ interface MatrixTimeline {
 
     suspend fun paginateBackwards(requestSize: Int, untilNumberOfItems: Int): Result<Unit>
     suspend fun fetchDetailsForEvent(eventId: EventId): Result<Unit>
+
+    suspend fun sendReadReceipt(eventId: EventId): Result<Unit>
 }