feat(core): add support config & button/error components

This adds a new app.support top level config that can be used to
configure various support links and other information. This can later be
extended for plugins to append their own specific support items.

app-config.yaml

@@ -2,6 +2,19 @@ app:
   title: Backstage Example App
   baseUrl: http://localhost:3000
   googleAnalyticsTrackingId: # UA-000000-0
+  support:
+    url: https://github.com/backstage/backstage/issues # Used by common ErrorPage
+    items: # Used by common SupportButton component
+      - title: Issues
+        icon: github
+        links:
+          - url: https://github.com/backstage/backstage/issues
+            title: GitHub Issues
+      - title: Discord Chatroom
+        icon: chat
+        links:
+          - url: https://discord.gg/MUpMjP2
+            title: '#backstage'
 
 backend:
   baseUrl: http://localhost:7000