Move or copy "Convert to draft" to PR’s mergeability box #3213

While "draft PRs" are kind of related to reviewers:

<img width="250" alt="Convert to draft in sidebar" src="https://user-images.githubusercontent.com/1402241/84258590-798b6f00-ab17-11ea-8157-7b8cd3f03fdb.png">

it's not where I expect to find the action, especially since "Ready for review" is in the mergeability box

<img width="929" alt="Ready for review in mergeability box" src="https://user-images.githubusercontent.com/1402241/84258719-af305800-ab17-11ea-9ebf-b068d0588a0a.png">

so I think it should copied or moved to the mergeability box, especially since https://github.com/sindresorhus/refined-github/pull/3211 makes room in it.

<img width="923" alt="" src="https://user-images.githubusercontent.com/1402241/84258819-dbe46f80-ab17-11ea-89a8-ee22c75b2303.png">

This seems easy to do: just copy the details element from the sidebar and replace the above element. This is what the sidebar's element looks like:

<details class="details-reset details-overlay d-inline">
    <summary role="button" class="btn-link muted-link">Convert to draft</summary>
    <details-dialog class="Box Box--overlay anim-fade-in fast" role="dialog" aria-modal="true">
      {et cetera}
    </details-dialog>
  </details>

A couple of additional changes need to be done too:

• remove .muted-link
• remove element once it's converted to draft; for some reason this doesn't happen like it happens for the sidebar. It's probably because the sidebar updates as a whole, implicitly removing the element. Probably this can be detected with onReplacedElement (it needs to be modified to have a second callback on "not found, exiting loop")