fix(invite): prefix a + when faking the validation response (#2597)

Pre-existing logic made it so numbers were assumed as valid
if no validation url was specified. To be consistent with
the validation server, the faked number should include a
+ at the beginning.

react/features/invite/functions.js

         // no auth url, let's say it is valid
         const response = {
             allow: true,
-            phone: dialNumber
+            phone: `+${dialNumber}`
         };
 
         return Promise.resolve(response);