Show a prompt to reinvite other party in a DM

If you are looking at a DM where the other party has left then
when you focus the composer for the first time we'll show a
dialog asking if you want to reinvite the other party.

Closes #590

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/room/MatrixRoom.kt

@@ -44,6 +44,7 @@ interface MatrixRoom : Closeable {
     val isEncrypted: Boolean
     val isDirect: Boolean
     val isPublic: Boolean
+    val activeMemberCount: Long
     val joinedMemberCount: Long
 
     /**