Remove hardcoded readthedocs.org URL
When using a custom readthedocs server, the search box in the documentation footer link to readthedocs.org website instead of the production domain host.
Using ``settings.PRODUCTION_DOMAIN`` fix the problem.
<dt>Search</dt>
<dd>
<div style="padding: 6px;">
- <form id="flyout-search-form" class="wy-form" target="_blank" action="https://readthedocs.org{% url 'elastic_project_search' project.slug %}" method="get">
+ <form id="flyout-search-form" class="wy-form" target="_blank" action="//{{ settings.PRODUCTION_DOMAIN }}{% url 'elastic_project_search' project.slug %}" method="get">
<input type="text" name="q" placeholder="Search docs">
</form>
</div>
