Create main template for Identity pages

Following conversation with @phamann and @gklopper:

- Create a new Identity main template (variant of `main.scala.html`)
- This shares the `<head>` but a different body structure - namely no ads and stripped down swipe containers. Note, these can't be entirely removed, some [dependencies remain](https://github.com/guardian/frontend/blob/identity-main/common/app/assets/javascripts/bootstraps/app.js#L107).
- Also creates a new Identity header template (variant of `header.scala.html`)
- This is a distraction-free version, removing the section navigation, top stories and search controls. Since the navigation styles are hooked on more than the `nav-container`, this also adds a `.has-nav` class to the existing header and styles in `_layout.scss`.