Fix wrong styling of todo items in exported HTML

The issue happened because styles connected with todo list were applied directly to HTML in Preview component. I added class to `li` tag of each todo item and style it from css.

browser/components/MarkdownPreview.js

@@ -360,10 +360,6 @@ export default class MarkdownPreview extends React.Component {
     }
     this.refs.root.contentWindow.document.body.innerHTML = markdown.render(value)
 
-    _.forEach(this.refs.root.contentWindow.document.querySelectorAll('.taskListItem'), (el) => {
-      el.parentNode.parentNode.style.listStyleType = 'none'
-    })
-
     _.forEach(this.refs.root.contentWindow.document.querySelectorAll('a'), (el) => {
       this.fixDecodedURI(el)
       el.addEventListener('click', this.anchorClickHandler)

browser/components/markdown.styl

@@ -178,6 +178,8 @@ ul
   margin-bottom 1em
   li
     display list-item
+    &.taskListItem
+      list-style none
     p
       margin 0
   &>li>ul, &>li>ol