add nsfw warning dialog

script/nsfw-warning.js

@@ -0,0 +1,35 @@
+(function () {
+'use strict';
+
+let nsfwOk = 'nsfw-ok';
+
+function alreadyYes() {
+  return sessionStorage.getItem(nsfwOk);
+}
+
+function dismiss() {
+  let dialog = document.getElementById('nsfw-dialog');
+  dialog.parentElement.removeChild(dialog);
+}
+
+function yes() {
+  sessionStorage.setItem(nsfwOk, 1);
+  dismiss();
+}
+
+function no() {
+  history.go(-1);
+}
+
+function setup() {
+  if (alreadyYes()) {
+    dismiss();
+  } else {
+    document.getElementById('nsfw-yes').onclick = yes;
+    document.getElementById('nsfw-no').onclick = no;
+  }
+}
+
+document.addEventListener('DOMContentLoaded', setup);
+
+})();