Accessibility: Add label to the Search Input Field

[nbertino]

The search input box could use a label to convey meaning to users with assistive devices.

Something like:

_layouts/website/page.html

{% block search_input %}
<div id="book-search-input" role="search">
    <label for="book-search-input-field" class="sr-only">{{ 'SEARCH_PLACEHOLDER'|t }}</label>
    <input id="book-search-input-field" type="text" placeholder="{{ 'SEARCH_PLACEHOLDER'|t }}" />
</div>
{% endblock %}

This would resolve a WCAG 2.0/AA error and make GitBook more accessible.

[rdsaunders]

@nbertino This change would be good to see.