Add config options to disable comments and the forum.

Add options to disable comments, the forum, and autocomplete. This is
for personal boorus and potentially for safe mode. Note that disabling
the forum may cause difficulties with creating and approving BURs.

Disabling comments and the forum merely hides them from most areas,
rather than completely removing them.

config/danbooru_default_config.rb

@@ -377,6 +377,21 @@ module Danbooru
       true
     end
 
+    # Whether to enable comments.
+    def comments_enabled?
+      true
+    end
+
+    # Whether to enable the forum.
+    def forum_enabled?
+      true
+    end
+
+    # Whether to enable autocomplete.
+    def autocomplete_enabled?
+      true
+    end
+
     def stripe_secret_key
     end