Add forum link to community dropdown (#151)

* Add forum link to community dropdown

Thought it might be good to add the forum link to the list.
When looking for previously answered questions, forum posts are easier to search through than slack.

* Change slack to forum

Author: 7ammer

layouts/_partials/header.ejs

@@ -77,6 +77,10 @@
                            target="_blank"
                            rel="noopener"
                            class="no-underline text-blue-lightest md:text-blue-dark hover:bg-green hover:text-white px-4 py-2">Slack</a>
+                        <a href="https://discourse.nativescript.org/c/vue"
+                           target="_blank"
+                           rel="noopener"
+                           class="no-underline text-blue-lightest md:text-blue-dark hover:bg-green hover:text-white px-4 py-2">Forum</a>
                     </div>
                 </div>
             </div>