Move all order logic to models

- Have a default order for each model
-- The overall default is ID DESC
- Allow for custom orderings
-- When comma-separated IDs are used

app/assets/javascripts/autocomplete.js.erb

@@ -341,7 +341,8 @@
       "filesize", "filesize_asc",
       "tagcount", "tagcount_asc",
       "rank",
-      "random"
+      "random",
+      "custom"
     ].concat(<%= TagCategory.short_name_list.map {|category| [category + "tags", category + "tags_asc"]}.flatten %>),
     status: [
       "any", "deleted", "active", "pending", "flagged", "banned"