Add migration for `en` to `en-US` language setting

Fixes #337

babybuddy/migrations/0020_update_language_en_to_en_us.py

@@ -0,0 +1,30 @@
+# Generated by Django 3.2.9 on 2021-12-13 21:25
+
+from django.db import migrations
+
+
+def update_language_en_to_en_us(apps, schema_editor):
+    Settings = apps.get_model('babybuddy', 'Settings')
+    for settings in Settings.objects.all():
+        if settings.language == 'en':
+            settings.language = 'en-US'
+            settings.save()
+
+
+def update_language_en_us_to_en(apps, schema_editor):
+    Settings = apps.get_model('babybuddy', 'Settings')
+    for settings in Settings.objects.all():
+        if settings.language == 'en-US':
+            settings.language = 'en'
+            settings.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('babybuddy', '0019_alter_settings_timezone'),
+    ]
+
+    operations = [
+        migrations.RunPython(update_language_en_to_en_us, reverse_code=update_language_en_us_to_en),
+    ]