Add meta description to pages where it is easy

Lots of the page types have obvious content that can be used as a meta
description. In those situations we should use it. We will have to think
harder about what to use in other situations.

Added a helper to make adding a meta description easy on any page.
Swap to using controller set meta descriptions

Setting them in the controller lets us write tests to assert that the
expected value is present. Having it in just the view made the testing
akward as we are trying to depricate using view_test.
Merge pull request #657 from alphagov/meta-description

Add meta description to pages where it is easy