Add official UN SDG icons

Note we're adding the icons in English on both the `sdg/en/` folder and
the `sdg/default/` folder. The latter stores the default icons when the
desired language does not have a fallback with an icon folder.

Also note we need to explicitely add the images to the asset pipeline
because they've been added to the `vendor/` folder; for some reason,
everything works properly without adding them to the asset pipeline if
we use the `app/` folder instead.

config/initializers/assets.rb

@@ -14,5 +14,7 @@ Rails.application.config.assets.precompile += %w[stat_graphs.js]
 Rails.application.config.assets.precompile += %w[dashboard_graphs.js]
 Rails.application.config.assets.precompile += %w[print.css]
 Rails.application.config.assets.precompile += %w[pdf_fonts.css]
+Rails.application.config.assets.precompile += %w[sdg/*.png]
+Rails.application.config.assets.precompile += %w[sdg/**/*.png]
 
 # Loads custom images and custom fonts before app/assets/images and app/assets/fonts