Added basic support for <datalist> elements for suggestions in Input widgets

[Volodymyr]

HTML5 introduces a support for <datalist> HTML5 elements for suggestions in input tag

To do this in Django currently, you have to do something like:

django/forms/widgets/datalist.html

{% include "django/forms/widgets/input.html" %}
<datalist id="{{ widget.id }}"{% include "django/forms/widgets/attrs.html" %}>{% for group_name, group_choices, group_index in widget.optgroups %}{% if group_name %}
  <optgroup label="{{ group_name }}">{% endif %}{% for option in group_choices %}
  {% include option.template_name with widget=option %}{% endfor %}{% if group_name %}
  </optgroup>{% endif %}{% endfor %}
</datalist>

django/forms/widgets/datalist_option.html

<option value="{{ widget.value|stringformat:'s' }}"{% include "django/forms/widgets/attrs.html" %}>

django/forms/widgets.py

class Datalist(Select):
    input_type = 'text'
    template_name = 'django/forms/widgets/datalist.html'
    option_template_name = 'django/forms/widgets/datalist_option.html'
    add_id_index = False
    checked_attribute = {'selected': True}
    option_inherits_attrs = False

django/forms/fields.py

class DatalistField(ChoiceField):
    widget = Datalist
    default_error_messages = {
        'invalid_choice': _('Select a valid choice. %(value)s is not one of the available choices.'),
    }

    def __init__(self, *, choices='', **kwargs):
        super().__init__(**kwargs)
        self.choices = choices