Make a reusable range input template.

[hmpf]

In order for any tick marks the widget must have a range to paint them in a for-loop. This is normally done via the context so we might have to sneak in something iterable into the form field that is automatically put in the context. Smells like a new FormField, frankly.

[podliashanyk]

I don't think we necessarily need a separate form field for this. We could use the original ChoiceFiled and pass choices=INCIDENT_LEVEL_CHOICES, with it. And then render those choices as a <datalist> of <option> below the range slider (to replace the hardcoded "12345").

[hmpf]

But then we need to override the html type to "range". So either way it's extra work.

[podliashanyk]

No extra work if this issue depends on #87, where you already render IntegerField as a NumberInput with a type set to range:

  maxlevel = forms.IntegerField(
        widget=forms.NumberInput(attrs={
            'type': 'range', 'step': '1', 'min': '1', 'max': MAX_INCIDENT_LEVEL
        }),
...