Make item height static in the sidebar

This fixes #4 but introduces a new bug where the HTML in the strings
in the sidebar is not converted to text anymore.

qml/Sidebar.qml

@@ -41,6 +41,7 @@ ScrollView {
             PostListItem {
                 textFontSize: item.textFontSize
                 nightmode: app.nightmode
+                width: listView.width
             }
         }