Add a shared searchable multiselect combobox for team/member pickers

Extracts the "Grant role" member picker's typeahead combobox into a
reusable static/js/searchable-select.js (opt in via data-searchable on
the widget), and reuses it for the news form's teams field -- a proper
multiselect with removable pills and filter-as-you-type, replacing the
plain checkbox list. Needed forwarding widget attrs through the shared
select template, which never passed them to the rendered <select>.

controlpanel/templates/templatetags/field.html

@@ -59,6 +59,7 @@
                         name="{{ field.html_name }}"
                         id="{{ field.auto_id }}"
                         {% if field.widget_type == "selectmultiple" %}multiple{% endif %}
+                        {% for attr, value in field.field.widget.attrs.items %}{{ attr }}="{{ value }}" {% endfor %}
                 >
 
                     {% if field.widget_type == "selectmultiple" %}