Create custom form field for choice-based filters

[jeremystretch]

Issue type

[x] Feature request
[ ] Bug report
[ ] Documentation

Environment

• Python version: 3.5.2
• NetBox version: 2.2.9

Description

NetBox currently employs custom functions to generate the options for choice-based fields in filter forms. For example, the following is for site status selection:

def site_status_choices():
    status_counts = {}
    for status in Site.objects.values('status').annotate(count=Count('status')).order_by('status'):
        status_counts[status['status']] = status['count']
    return [(s[0], '{} ({})'.format(s[1], status_counts.get(s[0], 0))) for s in SITE_STATUS_CHOICES]

class SiteFilterForm(BootstrapMixin, CustomFieldFilterForm):
    model = Site
    status = forms.MultipleChoiceField(choices=site_status_choices, required=False)
    ...

The logic used to calculate and attach the count of each option is replicated in the same manner for several fields. We should be able to devise a simple custom form field that can be re-used without duplicating this boilerplate code.