migrations: Add reverser for emoji_alt_code migration.

This is easy to do, and prevents this feature from getting a server
admin stuck in potentially a pretty uncomfortable way -- unable to
roll back a deploy.

zerver/migrations/0130_text_choice_in_emojiset.py

@@ -9,6 +9,17 @@ def change_emojiset(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> Non
         user.emojiset = "text"
         user.save(update_fields=["emojiset"])
 
+def reverse_change_emojiset(apps: StateApps,
+                            schema_editor: DatabaseSchemaEditor) -> None:
+    UserProfile = apps.get_model("zerver", "UserProfile")
+    for user in UserProfile.objects.filter(emojiset="text"):
+        # Resetting `emojiset` to "google" (the default) doesn't make an
+        # exact round trip, but it's nearly indistinguishable -- the setting
+        # shouldn't really matter while `emoji_alt_code` is true.
+        user.emoji_alt_code = True
+        user.emojiset = "google"
+        user.save(update_fields=["emoji_alt_code", "emojiset"])
+
 class Migration(migrations.Migration):
 
     dependencies = [
@@ -21,7 +32,7 @@ class Migration(migrations.Migration):
             name='emojiset',
             field=models.CharField(choices=[('google', 'Google'), ('apple', 'Apple'), ('twitter', 'Twitter'), ('emojione', 'EmojiOne'), ('text', 'Plain text')], default='google', max_length=20),
         ),
-        migrations.RunPython(change_emojiset),
+        migrations.RunPython(change_emojiset, reverse_change_emojiset),
         migrations.RemoveField(
             model_name='userprofile',
             name='emoji_alt_code',