[MM-19569] add notarization to osx app (#1086)

* [MM-19569] add notarization to release

also added signing verification on release
added electron-notarize to dev deps

* [MM-19569] fix semicolon

* [MM-19569] check that we are running on osx to notarize

package.json

@@ -55,6 +55,7 @@
     "electron": "^5.0.10",
     "electron-builder": "^21.2.0",
     "electron-connect": "^0.6.3",
+    "electron-notarize": "^0.1.1",
     "eslint": "^5.9.0",
     "eslint-config-mattermost": "github:mattermost/eslint-config-mattermost",
     "eslint-plugin-cypress": "^2.1.2",
@@ -77,4 +78,3 @@
     "webpack-merge": "^4.1.4"
   }
 }
-