FIX: Support `div dir=` in sanitizer

app/assets/javascripts/pretty-text/white-lister.js.es6

@@ -124,6 +124,7 @@ whiteListFeature('default', [
   'div.quote-controls',
   'div.title',
   'div[align]',
+  'div[dir]',
   'dl',
   'dt',
   'em',

test/javascripts/lib/sanitizer-test.js.es6

@@ -56,6 +56,8 @@ test("sanitize", function() {
   cooked(`<abbr title="JavaScript">JS</abbr>`, `<p><abbr title="JavaScript">JS</abbr></p>`);
   cooked(`<dl><dt>Forum</dt><dd>Software</dd></dl>`, `<dl><dt>Forum</dt><dd>Software</dd></dl>`);
   cooked(`<sup>high</sup> <sub>low</sub> <big>HUGE</big>`, `<p><sup>high</sup> <sub>low</sub> <big>HUGE</big></p>`);
+
+  cooked(`<div dir="rtl">RTL text</div>`, `<div dir="rtl">RTL text</div>`);
 });
 
 test("ids on headings", () => {