#552 Adds page_header_tabs block for extensions to add to tab lists

Looks more complicated than it really is. Change is: every place where we have tabs on top of the page now has a `{% block page_header_tabs %}` which allows extensions to:

    {% ckan_extends %}
    {% block page_header_tabs %}
      {{ super() }}
      {{ h.build_nav_icon('awesome_tab', _('Super awesome new tab')) }}
    {% endblock %}

It's not an ideal solution as not more than one extension can add to a tab list. But there is a solution that we need to write for 2.1