PERF: index on topic_id for user_histories table

We query this table when getting composer messages with the queries
such as:

```
SELECT  1 AS one FROM "user_histories"
WHERE "user_histories"."target_user_id" = 1 AND
 "user_histories"."action" = 9 AND
 "user_histories"."topic_id" = 105794
LIMIT 1
```

This index ensures this query remains very quick, regardless of user
history size.

db/migrate/20190106041015_add_topic_id_index_to_user_histories.rb

@@ -0,0 +1,5 @@
+class AddTopicIdIndexToUserHistories < ActiveRecord::Migration[5.2]
+  def change
+    add_index :user_histories, [:topic_id, :target_user_id, :action]
+  end
+end