New prev/next buttons with transition prompt

[andycochran]

This PR…

• Refactors the narrative's previous/next buttons with a better UI and call-to-action
• The hasNarrative config variable is renamed isLinearNarrative (because every map has a narrative, but only some are included in the linear previous/next flow)
• Removes the now-unused Foundation Pagination CSS
• Bigger next button, smaller previous button sans text (moving forward in the narrative is the primary action)
• Adds a transitionPrompt variable to maps in meta.yaml which allows for a phrase to act as a call-to-action for clicking the next button
• Adjusts the layout of narrative navigation, giving more padding where needed on various screen sizes

Closes #152.

[allthesignals]

looks great - pls merge. thanks!