Preserve SVG viewBox when minifying SVGs for production

By default svgo removes the `viewBox` attribute of the SVG, which breaks
scaling of the SVG content when rendering.

This caused various icons on the site to render at an incorrect scale.
In some cases this was not very noticeable because the natural size of
the content was close to the scaled size. In other cases the icon failed
to appear at all.

To see the difference, compare:

```
rm build/images/icons/*
NODE_ENV=production gulp build-images
cat build/images/icons/groups.svg
```

Before and after this patch is applied.

Fixes #5656

gulpfile.js

@@ -195,10 +195,23 @@ gulp.task('build-images', function() {
     return IS_PRODUCTION_BUILD && file.path.match(/\.svg$/);
   };
 
+  // See https://github.com/ben-eb/gulp-svgmin#plugins
+  var svgminConfig = {
+    plugins: [
+      {
+        // svgo removes `viewBox` by default, which breaks scaled rendering of
+        // the SVG.
+        //
+        // See https://github.com/svg/svgo/issues/1128
+        removeViewBox: false,
+      },
+    ],
+  };
+
   return gulp
     .src(imageFiles)
     .pipe(changed(IMAGES_DIR))
-    .pipe(gulpIf(shouldMinifySVG, svgmin()))
+    .pipe(gulpIf(shouldMinifySVG, svgmin(svgminConfig)))
     .pipe(gulp.dest(IMAGES_DIR));
 });