Allow specifying vote groups for rankings

Closes #19

aleksis/apps/abi/migrations/0017_rankingcategory_voter_groups.py

+# Generated by Django 3.2.16 on 2023-01-14 14:14
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('abi', '0016_rename_field_profilefieldvalue_profile_field'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='rankingcategory',
+            name='voter_groups',
+            field=models.ManyToManyField(blank=True, help_text='Only persons in these groups will be able to vote. Leave empty to allow all groups.', null=True, related_name='ranking_categories_voters', to='abi.AbiGroup', verbose_name='Voter Groups'),
+        ),
+    ]