Rework settings handling in sfu.js.

There is now a data structure that contains the ultimate truth
about the user's preferences.  We no longer grovel through the
UI components.

static/sfu.html

@@ -146,14 +146,12 @@
         <div id="mediaoptions">
           <label for="videoselect">Camera:</label>
           <select id="videoselect" class="select select-inline">
-            <option>default</option>
-            <option>off</option>
+            <option value="">off</option>
           </select>
 
           <label for="audioselect">Microphone:</label>
           <select id="audioselect" class="select select-inline">
-            <option>default</option>
-            <option>off</option>
+            <option value="">off</option>
           </select>
 
         </div>