Enable showdown's literalMidWordUnderscores

This stops showdown from interpreting underscores in the middle of words
as <em> and <strong>, instead it treats them as literal underscores.

Example:

some text with__underscores__in middle

will be parsed as

<p>some text with__underscores__in middle</p>

Fixes #2598 (URLs with _'s in them were getting <em> tags inserted into
them, breaking the links).

Also see <showdownjs/showdown#211>.

h/static/scripts/filter/converter.js

@@ -11,7 +11,12 @@ module.exports = function () {
   // see https://github.com/showdownjs/showdown#valid-options
   var converter = new showdown.Converter({
     extensions: [targetBlank],
-    simplifiedAutoLink: true
+    simplifiedAutoLink: true,
+    // Since we're using simplifiedAutoLink we also use
+    // literalMidWordUnderscores because otherwise _'s in URLs get
+    // transformed into <em>'s.
+    // See https://github.com/showdownjs/showdown/issues/211
+    literalMidWordUnderscores: true,
   });
   return converter.makeHtml.bind(converter);
 };

h/static/scripts/filter/test/converter-test.js

@@ -2,9 +2,19 @@ var converter = require('../converter');
 
 describe('markdown converter', function () {
   var markdownToHTML = converter();
+
   it('should autolink URLs', function () {
     assert.equal(markdownToHTML('See this link - http://arxiv.org/article'),
       '<p>See this link - <a target="_blank" href="http://arxiv.org/article">' +
       'http://arxiv.org/article</a></p>');
   });
+
+  it('should autolink URLs with _\'s in them correctly', function () {
+    assert.equal(
+      markdownToHTML(
+        'See this https://hypothes.is/stream?q=tag:group_test_needs_card'),
+      '<p>See this <a target="_blank" ' +
+      'href="https://hypothes.is/stream?q=tag:group_test_needs_card">' +
+      'https://hypothes.is/stream?q=tag:group_test_needs_card</a></p>');
+  });
 });