file (discover): new discovery page: the discovery page will show service, own devices and roster contact devices offering file sharing. An extra field allow to enter free jid. Added a small message in photo/discover.html has photo album discovery is not yes implemented. New block and images macros.

{% macro form(action='', class='') %}
<form method="post" action="{{action}}" {{ {'class': class}|xmlattr }}>
    {% if csrf_token is defined %}
        <input type="hidden" name="csrf_token" value="{{csrf_token}}">
    {% endif %}
    {{ caller() }}
</form>
{% endmacro %}