HIPPOTHEME-7: keep current page after livereload
HIPPOTHEME-7: make pane grid pane-index-based instead of pane-boundary based

Instead of, for example, .pane-xs-0-4 for a pane that stretches from "left-of-column-1" to "right-of-column-4", we can now simply use .pane-xs-1-4.

Also introduced '.pane-{breakpoint}-collapsed' for invisible panes from a certain breakpoint.
Merge branch 'master' of github.com:onehippo/hippo-theme