Fix tag combiner content type

This fixes a tag combiner bug I found through google for when a URL comes in with a tag `content/xyz`.
This is represented in the Content API as `type/xyz`, so it 404s currently.

For example, have your preference set to NGW, and search in google for `the guardian gallery`, and you will get a lot of 404 results, as they point to a tag with `content/gallery`.

Instead of transparently converting it, a long term solution would be to redirect it somehow. But this would probably happen after `R2`. (*gets switched off*)

@gklopper I could have just used regex here, but I don't like regex /much/.