Localization support for Index generation

[brew]

Following on from Index static site generation (#853), we also want to support localization.

Existing Metalsmith plugins for localization support:

• https://github.com/doup/metalsmith-multi-language
• https://github.com/macprog-guy/metalsmith-i18next

gettext and format functions are passed to templates in the generation process here:

opendatasurvey/index/generate.js

Lines 178 to 183 in 3febd32

	// Pass-through str for the moment. Can i18n-abide be used here?
	gettext: function(str) {
	return str;
	},
	// format function needs to be available in templates
	format: i18n.format,

Note: localization of the Index will be greatly informed by work done on the Survey (#844)