reaction: Fix missing unique constraint on Reactions model.

This fixes a missing unique constraint on the Reactions data model
state when using multiple aliases for an emoji code.  As with any
missing unique constraints, we first need to apply a migration that
eliminates violations of the rule; in this case, deleting the
duplicates is correct.

Added unique constraint for "user_profile", "message",
"reaction_type", "emoji_code".

Fixes #15347.

zerver/migrations/0288_reaction_unique_on_emoji_code.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2.13 on 2020-06-19 08:16
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0287_clear_duplicate_reactions'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='archivedreaction',
+            unique_together={('user_profile', 'message', 'emoji_name'), ('user_profile', 'message', 'reaction_type', 'emoji_code')},
+        ),
+        migrations.AlterUniqueTogether(
+            name='reaction',
+            unique_together={('user_profile', 'message', 'emoji_name'), ('user_profile', 'message', 'reaction_type', 'emoji_code')},
+        ),
+    ]