Use the singular form of tag type in tag URLs

Change the /tags/:tag_type/:tag_id.json endpoint to always expect the 
singular form of the tag ID. If the plural form of a known tag is 
provided, redirect to the tag URL containing the singular form.
Deprecate the /tags/<tag_type_or_id>.json endpoint

This endpoint was serving as a list of all tags for a particular type, 
something which /tags.json also does, but without extra functionality 
like sorting. It doesn't make sense to have two lots of code doing the 
same thing, so redirect requests here to /tags.json?type=<tag_type>.

Redirects for this endpoint's previous behaviour, where it would return 
a section tag with a matching ID (eg /tags/driving.json) remain in 
place and will continue to redirect to /tags/section/driving.json.
Fix typo in comment block
Accept strings for tag_id and tag_type in the tag_url helper

In the URLHelper class, change the tag_url method to accept a separate 
tag_id and tag_type, in addition to passing a Tag object.
Rename tag type variables to make them clearer
Add comment block describing tag_url method
Merge pull request #152 from alphagov/singular-tag-type-endpoints

Use the singular form of tag types in endpoints