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

webapp/sass/components/_tutorial.scss

@@ -183,7 +183,7 @@
             display: inline-block;
             margin-bottom: 50px;
             max-width: 310px;
-            min-height: 370px;
+            min-height: 410px;
             position: relative;
             text-align: left;
         }
@@ -234,3 +234,7 @@
         }
     }
 }
+
+.tutorial__app-icons {
+    margin: 2em 0;
+}