fix(eslint): Add no-alert rule

.eslintrc.js

         'dot-location': [ 'error', 'property' ],
         'dot-notation': 2,
         'guard-for-in': 2,
+        'no-alert': 2,
         'no-caller': 2,
         'no-case-declarations': 2,
         'no-div-regex': 0,

modules/RTC/ScreenObtainer.js

                 failCallback);
         } else {
             if (chromeExtUpdateRequired) {
+                /* eslint-disable no-alert */
                 alert(
                     'Jitsi Desktop Streamer requires update. '
                     + 'Changes will take effect after next Chrome restart.');
+                /* eslint-enable no-alert */
             }
 
             try {