Add Discord as chat instead of Gitter (#2878)

Author: Kludex

.github/ISSUE_TEMPLATE/config.yml

@@ -6,6 +6,6 @@ contact_links:
   about: >
     The "Discussions" forum is where you want to start. 💖
 - name: Chat
-  url: https://gitter.im/encode/community
+  url: https://discord.gg/SWU73HffbV
   about: >
     Our community chat forum.