Use UTC in a time-sensitive spec

* Using the offset string introduced two date-related spec failures
  when running outside the dev VM
* discovered during postgres-redux branch
Allow use of Time.utc

* this shouldn't break a build.
Add a link to a transition blog post to the definition of a mapping
Merge pull request #339 from alphagov/add_blog_post_link_to_mappings_glossary_entry

Add a link to a transition blog post to the definition of a mapping
Merge pull request #320 from alphagov/fix-timezone-spec-issue

Use UTC in a time-sensitive spec
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.