Disable selection globally on sidebar

We want to disable selections in the sidebar because when users drag
the handle, they could otherwise accidentally select stuff. This results
in a very broken state.

When selections are disabled, the sidebar also feels more like a GUI
element from a real application, and less like part of a webpage.

app/styles/base.css

@@ -246,6 +246,8 @@ html {
   background-color: rgb(110, 132, 163);
   border-radius: 0 10px 10px 0;
 
+  user-select: none;
+  -webkit-user-select: none;
 }
 #noVNC_control_bar.noVNC_open {
   box-shadow: 6px 6px 0px rgba(0, 0, 0, 0.5);