use per-server IrcChannel for reminders

this makes it so that if we have multiple bot instances running, they
will only pay attention to the countdown items for their current server

countdown/migrations/0007_countdownitem_reminder_target_new.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.1.2 on 2021-04-26 00:10
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('ircbot', '0018_ircserver_replace_irc_control_with_markdown'),
+        ('countdown', '0006_auto_20201025_1716'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='countdownitem',
+            name='reminder_target_new',
+            field=models.ForeignKey(blank=True, default=None, null=True, on_delete=django.db.models.deletion.CASCADE, to='ircbot.ircchannel'),
+        ),
+    ]