Do the basics to allow users to view all tags for a site they can see

- Add a route to /sites/:site_id/tags.
- Add a tags method to the SitesController to produce the maximum number
  of tags for a site - 200.
- Add a basic table iterating over each of the maximum of 200 tags.
Make each tag in the table a link to the mappings with that tag

- This could probably do with being <%= link_to ... %>, but I can't work
  out how to make it have ?tagged= querystrings.