js: Move dialogue-box sizing and positioning from JavaScript to CSS

This has the advantage that if you resize the window while a dialogue
box is active, the dialogue box adjusts itself accordingly.

html/jspage.pl

@@ -225,12 +225,15 @@ EOF
 }
 
 #dlgform {
+    width: 66.6667vw;
     opacity: 1;
     background: #ffffff;
     color: #000000;
     position: absolute;
     border: 2px solid black;
     padding: 20px;
+    top: 10vh;
+    left: 16.6667vw;
     z-index: 100;
 }