Remind the user if they're trying to reply to someone who has left

This happens semi-frequently to me and others - we reply to a user who
has left the room. Sometimes this is useful (for example, bringing up a
previous topic) but in most cases this is accidental, and almost
guaranteed to happen if you turned off join/leave events.

So I added a reminder when replying - which manifests as a small label
in the chatbar - that the user has left and can't be notified of your
reply.

src/chatbar/ChatBar.qml

@@ -197,6 +197,22 @@ QQC2.Control {
             visible: root.currentRoom.mainCache.replyId.length > 0
             sourceComponent: replyPane
         }
+        RowLayout {
+            visible: replyLoader.visible && !root.currentRoom.mainCache.relationAuthorIsPresent
+            spacing: Kirigami.Units.smallSpacing
+
+            Kirigami.Icon {
+                source: "help-hint-symbolic"
+                color: Kirigami.Theme.disabledTextColor
+
+                Layout.preferredWidth: Kirigami.Units.iconSizes.small
+                Layout.preferredHeight: Kirigami.Units.iconSizes.small
+            }
+            QQC2.Label {
+                text: i18nc("@info", "The user you're replying to has left the room, and can't be notified.")
+                color: Kirigami.Theme.disabledTextColor
+            }
+        }
         Loader {
             id: attachLoader