WagTail autocomplete for other models

[kokhoor]

Since when performing filter, the fieldname is either title or the autocomplete_search_field attribute, the render_page should be amended to add returning autocomplete values using autocomplete_search_field attribute of the record.

def render_page(page):
    if getattr(page, 'specific', None):
        # For support of non-Page models like Snippets.
        page = page.specific
    if callable(getattr(page, 'autocomplete_label', None)):
        title = page.autocomplete_label()
    elif getattr(page, 'autocomplete_search_field', None):  # added
        title = getattr(page, getattr(page, 'autocomplete_search_field', None))
    else:
        title = page.title
    return dict(pk=page.pk, title=title)