Clean up 404 page

• Move paragraph about the versions dropdown above the search bar

After a discussion with Mike, this paragraph was hidden by the search results, so I moved it before the search.

• Remove review app mention as the user would never see this

The following note was placed there in the first place to instruct users that were using review apps, in the case where the review app would not deploy. The thing is the users would never see this message, as the 404 page would never work before the site got deployed!

<% unless ENV['NANOC_ENV'] == 'production' %>
  <p class="gray">
    <em>Are you coming from a <strong>Review App</strong> when working on this site? Read the docs on
      <a href="https://docs.gitlab.com/ee/development/writing_documentation.html#troubleshooting-review-apps" >troubleshooting its review apps
      </a> for info on how to preview the page you're looking for.
    </em>
   </p>
  <br>
<% end %>