Update HTTP header config to always force reload of page

This is especially crucial since we were using Gatsby earlier;
browsers which still have the Gatsby PWA must be forced to refresh.

netlify.toml

@@ -1,3 +1,11 @@
 [build]
   publish = "dist"
-  command = "gridsome build"
+  command = "gridsome build"
+
+[[headers]]
+  for = "/*"
+  [headers.values]
+    cache-control = '''
+    public,
+    max-age=0,
+    must-revalidate'''