PLT-1866/PLT-3509 Added links to download the native apps (#3651)

* PLT-1866 Added configurable links for native app downloads

* PLT-1866 Added native app download link to main menu

* PLT-3509 Added native app link to tutorial

* PLT-3509 Added native app link to welcome email

* PLT-3509 Made link to apps in welcome email conditional on being set

config/config.json

@@ -195,5 +195,10 @@
         "NicknameAttribute": "",
         "LocaleAttribute": "",
         "LoginButtonText": "With SAML"
+    },
+    "NativeAppSettings": {
+        "AppDownloadLink": "https://about.mattermost.com/downloads/",
+        "AndroidAppDownloadLink": "https://about.mattermost.com/mattermost-android-app/",
+        "IosAppDownloadLink": "https://about.mattermost.com/mattermost-ios-app/"
     }
 }