Added min_num and max_num parameters to InlinePanel

[kaedroho]

This pull request implements min_num and max_num parameters on InlinePanel (#669)

It requires a change to django-modelcluster (wagtail/django-modelcluster#37)

[salvadormrf]

Can we disable the "add more" button on interface as well.

(function() {
    var panel = InlinePanel({
        formsetPrefix: "id_{{ self.formset.prefix }}",
        emptyChildFormPrefix: "{{ self.empty_child.form.prefix }}",
        canOrder: {% if can_order %}true{% else %}false{% endif %},
        maxForms: {{ self.formset.max_num }} // NEW
    });

    {% for child in self.children %}
        panel.initChildControls("{{ child.form.prefix }}");
    {% endfor %}
    panel.setHasContent();
    panel.updateMoveButtonDisabledStates();
    panel.updateAddButtonState(); // NEW
})();

function InlinePanel(opts) {
....
    self.updateAddButtonState = function() {  // NEW
        if (opts.maxForms) {
            var currentforms = $('.inline-form', self.formsContainer).not('.deleted').length;
            var addButton = $('#' + opts.formsetPrefix + '-ADD');

            (currentforms >= opts.maxForms) ? addButton.addClass("disabled") : addButton.removeClass("disabled");
        }
    };
...
}

[nimasmi]

I hereby notify of my appreciation and anticipation of this feature, in an attempt to alert you to the benefits it might have to others, and to convince you to prioritise it (otherwise known as 👍ing).

This applies to a really common situation from front-enders' perspectives, where related items are listed horizontally, but a layout would break if more than n were used. My current way of implementing something that achieves the same is the exceedingly ugly:

related_item_1 = models.ForeignKey('wagtailcore.Page', null=True, related_name='+', blank=True, on_delete=models.SET_NULL)
related_item_2 = models.ForeignKey('wagtailcore.Page', null=True, related_name='+', blank=True, on_delete=models.SET_NULL)
related_item_3 = models.ForeignKey('wagtailcore.Page', null=True, related_name='+', blank=True, on_delete=models.SET_NULL)
related_item_4 = models.ForeignKey('wagtailcore.Page', null=True, related_name='+', blank=True, on_delete=models.SET_NULL)