reactions: Migrate emoji_code to store id for realm emoji.

Till now, we had been storing realm emoji's name in emoji code field
in reactions' model. This commit migrates it to store realm emoji's id.
It is a part of effort to migrate realm emojis to be referenced by their
id and not by name.

zerver/migrations/0145_reactions_realm_emoji_name_to_id.py

@@ -0,0 +1,51 @@
+# -*- coding: utf-8 -*-
+
+import ujson
+
+from collections import defaultdict
+from django.conf import settings
+from django.db import migrations, models
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+from typing import Any, Dict
+
+def realm_emoji_name_to_id(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    Reaction = apps.get_model('zerver', 'Reaction')
+    RealmEmoji = apps.get_model('zerver', 'RealmEmoji')
+    realm_emoji_by_realm_id = defaultdict(dict)   # type: Dict[int, Dict[str, Any]]
+    for realm_emoji in RealmEmoji.objects.all():
+        realm_emoji_by_realm_id[realm_emoji.realm_id] = {
+            'id': str(realm_emoji.id),
+            'name': realm_emoji.name,
+            'deactivated': realm_emoji.deactivated,
+        }
+    for reaction in Reaction.objects.filter(reaction_type='realm_emoji'):
+        realm_id = reaction.user_profile.realm_id
+        emoji_name = reaction.emoji_name
+        if realm_id in realm_emoji_by_realm_id:
+            realm_emoji = realm_emoji_by_realm_id[realm_id].get(emoji_name)
+            if realm_emoji is None:
+                # Realm emoji used in this reaction has been deleted so this
+                # reaction should also be deleted. We don't need to reverse
+                # this step in migration reversal code.
+                reaction.delete()
+            else:
+                reaction.emoji_code = realm_emoji["id"]
+                reaction.save()
+
+def reversal(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    Reaction = apps.get_model('zerver', 'Reaction')
+    for reaction in Reaction.objects.filter(reaction_type='realm_emoji'):
+        reaction.emoji_code = reaction.emoji_name
+        reaction.save()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0144_remove_realm_create_generic_bot_by_admins_only'),
+    ]
+
+    operations = [
+        migrations.RunPython(realm_emoji_name_to_id,
+                             reverse_code=reversal),
+    ]