Added `can? :create, Item` guard on "Add Item" link (fixes #776)
Added `can? :create, Attachment` guard on "Add Attachment" link
Added `can? :create, Component` guard on "Add Component" link
Limited create menu options to AdminPolicy and Collection
Merge pull request #777 from duke-libraries/776-add-item-link

Added `can? :create, Item` guard on "Add Item" link (fixes #776)