feat(ChatRoom): add 'disableFocus' option

Adds a flag which can be used to disable talking to Jicofo. It's
intended to be used only in jitsi-meet-spot which joins a special room.

modules/xmpp/ChatRoom.js

      * @param password
      * @param XMPP
      * @param options
+     * @param {boolean} options.disableFocus - when set to {@code false} will
+     * not invite Jicofo into the room. This is intended to be used only by
+     * jitsi-meet-spot.
      */
     constructor(connection, jid, password, XMPP, options) {
         super();
         this.password = password;
 
         return new Promise(resolve => {
-            this.moderator.allocateConferenceFocus(() => {
+            this.options.disableFocus
+                && logger.info('Conference focus disabled');
+
+            const preJoin
+                = this.options.disableFocus
+                    ? Promise.resolve()
+                    : this.moderator.allocateConferenceFocus();
+
+            preJoin.then(() => {
                 this.sendPresence(true);
                 resolve();
             });

modules/xmpp/moderator.js

  *
  * @param {Function} callback - the function to be called back upon the
  * successful allocation of the conference focus
+ * @returns {Promise} - Resolved when Jicofo allows to join the room. It's never
+ * rejected and it'll keep on pinging Jicofo forever.
  */
-Moderator.prototype.allocateConferenceFocus = function(callback) {
-    // Try to use focus user JID from the config
-    this.setFocusUserJid(this.options.connection.focusUserJid);
+Moderator.prototype.allocateConferenceFocus = function() {
+    return new Promise(resolve => {
+        // Try to use focus user JID from the config
+        this.setFocusUserJid(this.options.connection.focusUserJid);
 
-    // Send create conference IQ
-    this.connection.sendIQ(
-        this.createConferenceIq(),
-        result => this._allocateConferenceFocusSuccess(result, callback),
-        error => this._allocateConferenceFocusError(error, callback));
-
-    // XXX We're pressed for time here because we're beginning a complex and/or
-    // lengthy conference-establishment process which supposedly involves
-    // multiple RTTs. We don't have the time to wait for Strophe to decide to
-    // send our IQ.
-    this.connection.flush();
+        // Send create conference IQ
+        this.connection.sendIQ(
+            this.createConferenceIq(),
+            result => this._allocateConferenceFocusSuccess(result, resolve),
+            error => this._allocateConferenceFocusError(error, resolve));
+
+        // XXX We're pressed for time here because we're beginning a complex
+        // and/or lengthy conference-establishment process which supposedly
+        // involves multiple RTTs. We don't have the time to wait for Strophe to
+        // decide to send our IQ.
+        this.connection.flush();
+    });
 };
 
 /**
 
     // Reset response timeout
     this.getNextTimeout(true);
-    window.setTimeout(() => this.allocateConferenceFocus(callback), waitMs);
+    window.setTimeout(
+        () => this.allocateConferenceFocus().then(callback),
+        waitMs);
 };
 
 /**
         const waitMs = this.getNextTimeout();
 
         logger.info(`Waiting for the focus... ${waitMs}`);
-        window.setTimeout(() => this.allocateConferenceFocus(callback),
+        window.setTimeout(
+            () => this.allocateConferenceFocus().then(callback),
             waitMs);
     }
 };