Tag and Section pages have now been replaced by IndexPage

Basic (non customised) tag and seciotn pages have almost exactly the same layout. I have merged them down into a single IndexPage controller and template.

This has the added benefit that it supports sections with 'tag like' urls e.g. http://www.theguardian.com/teacher-network/zurich-school-competition.

This is quite a large changeset, mostly due to renaming things.