Split out partials

Renames the main template to reflect it being based on [govuk_template](https://github.com/alphagov/govuk_template) and splitting all the project-specific areas of it into [template tags](https://www.playframework.com/documentation/1.2.4/templates#anametagsCustomtemplatetagsa). 

This also splits the HTML wrapping the main content block into `layout.html` to let us define separate templates for the different types of page.

In future, we should move the `govuk_template.html` file to use a [Groovy](http://groovy.codehaus.org/) version of the [govuk_template](https://github.com/alphagov/govuk_template). I have used the same template variable names here to enable that.