Make 'skip to main content' skip breadcrumbs by moving them to a content_for.

I was looking for a way to move the breadcrumbs out of #main, so that the accessible 'skip to main content' link would skip past them.

I noticed this `:context_bar` block was added to `app/views/layouts/_unconstrained.html.erb` recently (by @alexwllms), so I moved the breadcrumbs partial render into that.

If you had other plans for it, let me know.