Display help message after receiving permissions from server.

We used to display the help message too early, which caused us to
display it even if the group doesn't exist or the user doesn't have
the present permission.

static/sfu.js

@@ -87,8 +87,6 @@ function setConnected(connected) {
         userbox.classList.remove('invisible');
         connectionbox.classList.add('invisible');
         displayUsername();
-        displayMessage("Press Present to enable your camera or microphone",
-                      "info");
     } else {
         resetUsers();
         let userpass = getUserPass();
@@ -770,6 +768,9 @@ function clearUsername() {
 function gotPermissions(perms) {
     displayUsername();
     setButtonsVisibility();
+    if(serverConnection.permissions.present)
+        displayMessage("Press Present to enable your camera or microphone",
+                       "info");
 }
 
 const urlRegexp = /https?:\/\/[-a-zA-Z0-9@:%/._\\+~#=?]+[-a-zA-Z0-9@:%/_\\+~#=]/g;