webpack: handle _global img folder

Fixes missing image files after composer cleaning assets

Source of requireAll(): https://stackoverflow.com/a/30652110

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>

app/Resources/static/themes/_global/share.js

@@ -1 +1,4 @@
 import './share.scss';
+
+function requireAll(r) { r.keys().forEach(r); }
+requireAll(require.context('./img/', true, /\.(jpg|png|gif|svg|ico)$/));