layout-header

Add default header to layout for public pages.

Fixes #43

Screenshot of default header:
https://www.evernote.com/shard/s7/sh/148ba858-145f-4c0d-993a-e43d873ebd69/ac21b6dccde021406bb06e63a617ff15

Changes:
* Add content_for :header to configure custom header per page
* Update home page to "opt-out" of default header rendering
* combine _navigation and _navigation_links partials into _header

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