Fix amd-inject-loader for Webpack 4

[MAILPOET-1667]

assets/js/src/amd-inject-loader-fixed.js

@@ -0,0 +1,6 @@
+var amdInjectLoader = require('amd-inject-loader');
+
+// fix original 'amd-inject-loader' for Webpack 4 by adding missing 'options' field to its context
+module.exports = function amdInjectLoaderFixed(input) {
+  return amdInjectLoader.call(Object.assign({}, this, { options: {} }), input);
+};