Fix Bug 1016500 - Implement Manifesto redesign

Do not merge yet. Still needs photo usage permissions and l10n.
The semantics are somewhat confusing here, here are my thoughts:
* the close element should be found last in the dialog
* the close element is now both a `<a>` and a `<button>` it should only be a `<button>` with a more descriptive text. eg. "Close dialog" where "dialog" can be visually-hidden. and the "x" can be appended via an image or similar, just don't go into the [multiplication x](https://www.youtube.com/watch?v=QfVVaU8EMEE) trap
* just as @alexgibson says `<nav>` should be reserved for major navigation blocks, so that's not correct to use here. `role="toolbar"` is mostly used in conjunction with `role="application"` and I think it will mostly confuse people in this situation. I would say a unsemantic `<div>` is the best choice here
* I think you should switch place with the next and previous button so that the next button is first (it's a more natural reading order). The result will be "show next principal", "show previous principal", "close dialog"
* "**show** next/previous principle" is a more understandable button text.
* When pressing the next and previous button then the `focus` should be moved to `#modal` as it does when you first open the dialog