Reset the lastMessageHasAlternateBackground{Reverse} values when clearing all messages in a ChannelView. (#1810)

This makes it so that we consistently start with the same values after
having cleared a ChannelView.
Example place where this happens: UserInfoPopup when user messages are
loaded and refreshed

src/widgets/helper/ChannelView.cpp

@@ -467,6 +467,9 @@ void ChannelView::clearMessages()
     this->messages_.clear();
     this->scrollBar_->clearHighlights();
     this->queueLayout();
+
+    this->lastMessageHasAlternateBackground_ = false;
+    this->lastMessageHasAlternateBackgroundReverse_ = true;
 }
 
 Scrollbar &ChannelView::getScrollBar()