Switch to using content_id in tag URLs

This changes the URL parameters for the Tag model and the TagsController
to use the 'content_id' field instead of the database 'id'. This will
make it easier to build a link to edit a specific tag in the future.
Show a helpful message when no tags/children exist

This changes the table which lists tags so that, when the collection
contains no tags, we show a helpful message, and a link to create a
new tag.