Add method find_by_slug_or_id! to Sluggable module

Make it easier to find by slug or id for sluggable models. It will
raise a 404 HTML Not found error if the resource is not found.

app/models/concerns/sluggable.rb

@@ -7,6 +7,10 @@ module Sluggable
     def self.find_by_slug_or_id(slug_or_id)
       find_by_slug(slug_or_id) || find_by_id(slug_or_id)
     end
+
+    def self.find_by_slug_or_id!(slug_or_id)
+      find_by_slug(slug_or_id) || find(slug_or_id)
+    end
   end
 
   def generate_slug