Create section to add destroy action to `tags_controller.rb`.

Toward the end of section I5 where we are sprinkling `if logged_in?`'s there is a line that says: 

    Similarly, hide the ‘delete’ link for the tags index.

But in section "I3: Tagging" we never created the `delete` link.
wrapped lines to 80 col
Merge pull request #647 from oblakeerickson/patch-6

Create section to add destroy action to `tags_controller.rb`.