Restore checkbox rendering and prevent poor sanitization of spans (#11277)

* Add test

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Restore checkbox rendering and prevent poor sanitization of spans

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Also fix preview context

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Also fix preview context

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/markup/markdown/markdown_test.go

@@ -140,6 +140,12 @@ func testAnswers(baseURLContent, baseURLImages string) []string {
 </ol>
 <h2 id="user-content-custom-id">More tests</h2>
 <p>(from <a href="https://www.markdownguide.org/extended-syntax/" rel="nofollow">https://www.markdownguide.org/extended-syntax/</a>)</p>
+<h3 id="user-content-checkboxes">Checkboxes</h3>
+<ul class="task-list">
+<li><span class="ui checkbox"><input type="checkbox" readonly="readonly"/><label>unchecked</label></span></li>
+<li><span class="ui checked checkbox"><input type="checkbox" checked="" readonly="readonly"/><label>checked</label></span></li>
+<li><span class="ui checkbox"><input type="checkbox" readonly="readonly"/><label>still unchecked</label></span></li>
+</ul>
 <h3 id="user-content-definition-list">Definition list</h3>
 <dl>
 <dt>First Term</dt>
@@ -207,6 +213,12 @@ Here are some links to the most important topics. You can find the full list of
 
 (from https://www.markdownguide.org/extended-syntax/)
 
+### Checkboxes
+
+- [ ] unchecked
+- [x] checked
+- [ ] still unchecked
+
 ### Definition list
 
 First Term