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

utils/config.go

@@ -264,6 +264,10 @@ func getClientConfig(c *model.Config) map[string]string {
 	props["RestrictCustomEmojiCreation"] = *c.ServiceSettings.RestrictCustomEmojiCreation
 	props["MaxFileSize"] = strconv.FormatInt(*c.FileSettings.MaxFileSize, 10)
 
+	props["AppDownloadLink"] = *c.NativeAppSettings.AppDownloadLink
+	props["AndroidAppDownloadLink"] = *c.NativeAppSettings.AndroidAppDownloadLink
+	props["IosAppDownloadLink"] = *c.NativeAppSettings.IosAppDownloadLink
+
 	if IsLicensed {
 		if *License.Features.CustomBrand {
 			props["EnableCustomBrand"] = strconv.FormatBool(*c.TeamSettings.EnableCustomBrand)