Added tags to images using acts_as_taggable_on gem

* Add acts_as_taggable_on to Gemfile
* Generate migrations for acts_as_taggable_on
* Update image model to be taggable
* Display an image's tags on the image show view
Add ability to tag/untag images and view tags

* Add field to image form partial for tags
* Update strong params in images_controller for tags
* Link to tag show view from images show view
* Add route for tags#show
* Create tag show view to display all images with a given tag
Refactor tags to use top-level route