Pages with multiple selectors have items with duplicated IDs

[flodolo]

Noticed in /messaging, but it's the same in /admin.

We use multiple instance of the widget multiple_item_selector, which uses input fields with IDs.

<div class="multiple-item-selector clearfix">
    {{ item_selector_list.list(
        id='available',
        title='Available',
        items=items_available,
        right_shortcut='Move all →',
    )}}
    {{ item_selector_list.list(
        id='selected',
        title='Chosen',
        items=items_selected,
        right_shortcut='← Move all',
        sortable=sortable,
    )}}
    {% if form_field %}
    {{ form_field }}
    {% endif %}
</div>