Fix issue markdown bugs (#17411)

* Bug fix: render Markdown http://AppURL/org/repo/issues/4?a=1&b=2#comment-123 test to HTML correctly, close #17394
* Bug fix: fix the positions of checkboxes in rendered HTML, close #17395

web_src/less/markup/content.less

@@ -158,9 +158,12 @@
 
   .task-list-item {
     list-style-type: none;
+    position: relative;
 
     input[type="checkbox"] {
-      margin: 0 6px .25em -1.6em;
+      position: absolute;
+      top: .25em;
+      left: -1.6em;
     }
   }