Bug 994831 - update history page

This resurrects the 15th anniversary slideshow and moves the old history content to a new page (with some copy updates).

L10n alert: The slide show had been localized in several languages but this update does change two slides. We don't really have any fallback content to display for other locales so I don't think an l10n tag will work here. But most of the strings were translated before so l10n has something of a head start and we can just activate locales as they're completed. For the moment I'm loading in the mozorg/15years lang files but we should perhaps rename/reorganize that? Thoughts from @flodolo and @pascalchevrel?

The history page isn't nearly as prominent as /contribute is so I think we're okay with launching en-US first. The details page probably isn't a priority for l10n, and I don't think the old history page was ever localized anyway.