Checklist nesting

[emileighoutlaw]

Makes hierarchical improvements to checklist content. Clarifies relationships of parent and child pages.

Resolves: https://github.com/18F/fec-cms/issues/408#issuecomment-235633285

[codecov-io]

Current coverage is 90.17% (diff: 100%)

Merging #413 into develop will not change coverage

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[noahmanger]

Nice! Just made a small change: when you're nesting a <ul> inside another <ul> it should be in one of the <li> elements. So:

<ul>
  <li>item</li>
  <li>items:
    <ul>
      <li>item</li>
      <li>item</li>
     </ul>
   </li>
</ul>