Add a feature flag and xmpp-caps entry for joining as a visitor. (#2247)

* Add a feature flag and xmpp-caps entry for joining as a visitor.

* Default to enabled, use for the confernece-request flag too.

modules/flags/FeatureFlags.js

      * @param {object} flags - The feature flags.
      * @param {boolean=} flags.runInLiteMode - Enables lite mode for testing to disable media decoding.
      * @param {boolean=} flags.ssrcRewritingEnabled - Use SSRC rewriting. Requires sourceNameSignaling to be enabled.
+     * @param {boolean=} flags.enableJoinAsVisitor - Enable joining as a visitor.
      */
     init(flags) {
         this._runInLiteMode = Boolean(flags.runInLiteMode);
         this._ssrcRewriting = Boolean(flags.ssrcRewritingEnabled);
+        this._joinAsVisitor = Boolean(flags.enableJoinAsVisitor ?? true);
     }
 
     /**
     isSsrcRewritingSupported() {
         return this._ssrcRewriting;
     }
+
+    /**
+     * Checks if the clients supports joining as a visitor.
+     * @returns {boolean}
+     */
+    isJoinAsVisitorSupported() {
+        return this._joinAsVisitor;
+    }
 }
 
 export default new FeatureFlags();

modules/xmpp/moderator.js

 import $ from 'jquery';
 import { $iq, Strophe } from 'strophe.js';
 
+import FeatureFlags from '../flags/FeatureFlags';
 import Settings from '../settings/Settings';
 
 const AuthenticationEvents
                 }).up();
         }
     }
-    elem.c('property', {
-        name: 'visitors-version',
-        value: 1 }).up();
+    if (FeatureFlags.isJoinAsVisitorSupported()) {
+        elem.c('property', {
+            name: 'visitors-version',
+            value: 1 }).up();
+    }
 
     return elem;
 };

modules/xmpp/xmpp.js

         if (FeatureFlags.isSsrcRewritingSupported()) {
             this.caps.addFeature('http://jitsi.org/ssrc-rewriting-1');
         }
+
+        // Use "-1" as a version that we can bump later. This should match
+        // the version added in moderator.js, this one here is mostly defined
+        // for keeping stats, since it is not made available to jocofo at
+        // the time of the initial conference-request.
+        if (FeatureFlags.isJoinAsVisitorSupported()) {
+            this.caps.addFeature('http://jitsi.org/visitors-1');
+        }
     }
 
     /**