Add hackaround for Qt bug to all non-horizontally-scrollable scrollviews

https://bugreports.qt.io/browse/QTBUG-83890 has been open for years with
a patch that's been stalled for years. There's no indication that it's
going to be fixed anytime soon, and it generates bug reports for us.
Let's add the typical hackaround for all non-horizontally-scrollable
scrollviews.

BUG: 457584

imports/NeoChat/Menu/Timeline/MessageSourceSheet.qml

@@ -22,6 +22,10 @@ Kirigami.Page {
     ScrollView {
         anchors.fill: parent
         contentWidth: availableWidth
+
+        // HACK: Hide unnecessary horizontal scrollbar (https://bugreports.qt.io/browse/QTBUG-83890)
+        ScrollBar.horizontal.policy: ScrollBar.AlwaysOff
+
         TextArea {
             id: sourceTextArea
             text: sourceText