Make page for redirecting to Discord server invite URL

Pretty much same reasons as for 349a6e3da3

This allows us to have one link we can use for linking to the Discord,
which can be easily updated if e.g., a new invite URL has to be
generated. Instead of Nexus, forum thread/topic, etc., etc. needing to
be updated, now they can all link to https://loot.github.io/discord/ and
this page should always point the right way.

_layouts/page.html

@@ -46,7 +46,7 @@
                     <i class="material-icons">forum</i>
                     <span>Forum Thread</span>
                 </a>
-                <a class="mdl-navigation__link mdl-navigation__link--icon" href="https://discord.gg/SZVPRzf" target="_blank">
+                <a class="mdl-navigation__link mdl-navigation__link--icon" href="/discord/" target="_blank">
                     <i class="material-icons">comment</i>
                     <span>Discord Server</span>
                 </a>

discord.html

@@ -0,0 +1,12 @@
+---
+layout: page
+title: Discord Server
+permalink: /discord/
+redirect_to:
+  - https://discord.gg/SZVPRzf
+discord_server: https://discord.gg/SZVPRzf
+---
+
+<p>The official LOOT Discord server can be found at:
+    <a href="{{ page.forum_thread }}">{{ page.forum_thread }}</a>
+</p>