Convert codebase to using ItemSlug

* Flesh out the formerly incomplete ItemSlug class
* Convert all uses of text item slugs to use ItemSlug
* Move the ItemSlug class under models where it properly belongs
* Avoid the use of `slug` as a local variable or item attribute
  wherever possible to avoid confusion between various kinds of slugs
* Merge the InventoryParser class into the Inventory class as it is
  unlikely to need to support multiple versions
* Simplify the ModPack/Mod/ModVersion interfaces by replacing the use
  of various "helper" methods with equivalent use of more generic
  methods
* Update the various methods of ModPack/Mod/ModVersion to be able to
  work with qualified or unqualified ItemSlugs
* Remove the unused BaseCollection class
* Add tests for ItemSlug and update all other tests to reflect
  all the other changes

src/scripts/controllers/stack_controller.coffee

@@ -49,7 +49,6 @@ module.exports = class StackController extends BaseController
         @$nameLink.html display.itemName
         @$nameLink.attr 'href', display.itemUrl
         @$quantityField.html @model.quantity
-        # @$removeButton.css opacity:(if @editable then 1 else 0)
 
         @$action.css display:(if @editable then 'table-cell' else 'none')