option to replace IRC control chars with markdown

^C^B isn't allowed through Discord's API, and I'm sure some other stuff
like colors that I don't use. this makes it a server option to replace
them with Markdown, though I think this would only ever be interesting
for BitlBee + Discord

ircbot/migrations/0018_ircserver_replace_irc_control_with_markdown.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.1.2 on 2021-04-25 17:05
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('ircbot', '0017_ircchannel_server'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='ircserver',
+            name='replace_irc_control_with_markdown',
+            field=models.BooleanField(default=False),
+        ),
+    ]