Move slack link use a nicer redirect link, so only needs to be updated once

Author: Relequestual

_includes/footer.html

@@ -12,7 +12,7 @@ <h2 class="footer-heading">{{ site.title | escape }}</h2>
             {% include icon-github.html username=site.github_username %}
           </li>
           {% endif %}
-          <li>Discussion: <a href="https://join.slack.com/t/json-schema/shared_invite/enQtNjc5NTk0MzkzODg5LTVlZGIxNmVhMGY2MWFlYTdiNDQ5NWFiZGUwOThhNmYxZDE0YzA5YjRiOTA5MGY4ZTZlZGZhZDFmYTY4NWM2N2Y">Slack</a> | <a href="https://groups.google.com/forum/#!forum/json-schema">Google Groups</a></li>
+          <li>Discussion: <a href="/slack">Slack</a> | <a href="https://groups.google.com/forum/#!forum/json-schema">Google Groups</a></li>
           <li>Site edits: <a href="https://github.com/json-schema-org/json-schema-org.github.io">GitHub repo for site</a></li>
         </ul>
       </div>

_includes/header.html

@@ -25,7 +25,7 @@
             <a class="page-link" href="{{ my_page.url | relative_url }}">{{ my_page.title | escape }}</a>
             {%- endif -%}
           {% endfor -%}
-          <a class="page-link" href="https://join.slack.com/t/json-schema/shared_invite/enQtNjc5NTk0MzkzODg5LTVlZGIxNmVhMGY2MWFlYTdiNDQ5NWFiZGUwOThhNmYxZDE0YzA5YjRiOTA5MGY4ZTZlZGZhZDFmYTY4NWM2N2Y">Join our Slack</a>
+          <a class="page-link" href="/slack">Join our Slack</a>
         </div>
       </nav>
     {%- endif -%}

index.md

@@ -32,7 +32,7 @@ After that, you could check out all our [learning resources](/learn).
 
 We're here to help!
 
-At any point, feel free to join our Slack server using the link in the menu.
+At any point, feel free to join our [Slack server](/slack).
 
 We also monitor the `jsonschema` tag on StackOverflow.
 
@@ -111,4 +111,4 @@ Questions? Feeling helpful? Get involved on:
 
 * [GitHub](http://github.com/json-schema-org/json-schema-spec)
 * [Google Groups](https://groups.google.com/forum/#!forum/json-schema)
-* [Slack](https://join.slack.com/t/json-schema/shared_invite/enQtNjc5NTk0MzkzODg5LTVlZGIxNmVhMGY2MWFlYTdiNDQ5NWFiZGUwOThhNmYxZDE0YzA5YjRiOTA5MGY4ZTZlZGZhZDFmYTY4NWM2N2Y)
+* [Slack](/slack)

slack.md

@@ -0,0 +1,3 @@
+---
+redirect_to: https://join.slack.com/t/json-schema/shared_invite/enQtNjc5NTk0MzkzODg5LTVlZGIxNmVhMGY2MWFlYTdiNDQ5NWFiZGUwOThhNmYxZDE0YzA5YjRiOTA5MGY4ZTZlZGZhZDFmYTY4NWM2N2Y
+---