feat(call-flows): Add config property for enabling call flows. (#3031)

react/features/base/config/functions.js

     'autoRecord',
     'autoRecordToken',
     'avgRtpStatsN',
+    'callFlowsEnabled',
     'callStatsConfIDNamespace',
     'callStatsID',
     'callStatsSecret',

react/features/invite/actions.js

 
         const state = getState();
         const { conference } = state['features/base/conference'];
-        const { inviteServiceUrl } = state['features/base/config'];
+        const {
+            callFlowsEnabled,
+            inviteServiceUrl,
+            inviteServiceCallFlowsUrl
+        } = state['features/base/config'];
         const inviteUrl = getInviteURL(state);
         const { jwt } = state['features/base/jwt'];
 
             // filter all rooms and users from {@link invitesLeftToSend}.
             const peopleInvitePromise
                 = invitePeopleAndChatRooms(
-                    inviteServiceUrl,
+                    callFlowsEnabled
+                        ? inviteServiceCallFlowsUrl : inviteServiceUrl,
                     inviteUrl,
                     jwt,
                     usersAndRooms)