These icons have been downloaded from The Global Goals page [1]. English
is the official language of this page and the only one containing all
the information.
Since SVG icons are smaller and can be compressed, users browsing the
page in English will have to download about 45KB for the SDG icons,
instead of the 250KB they needed to download when using PNGs.
[1] https://globalgoals.org
The objective of this change is to reduce the size of the images to improve the
experience in slow connections, poor coverage situations or data limits so common in
mobile devices.
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.
