Optimize heatmap query (#33853)

When there are over 5M records on `action` table, the heatmap on
dashboard is very slow as below SQL.
```
database duration=1.8881s db.sql="SELECT created_unix DIV 900 * 900 AS timestamp, count(user_id) as contributions FROM `action` WHERE user_id=? AND act_user_id=? AND (created_unix > ?) GROUP BY timestamp ORDER BY timestamp"
```

This PR add a new index for `action` table with columns `user_id`,
`act_user_id` and `created_unix` so that this query will become about 6
times faster than before.

models/migrations/migrations.go

@@ -377,6 +377,7 @@ func prepareMigrationTasks() []*migration {
 		newMigration(314, "Update OwnerID as zero for repository level action tables", v1_24.UpdateOwnerIDOfRepoLevelActionsTables),
 		newMigration(315, "Add Ephemeral to ActionRunner", v1_24.AddEphemeralToActionRunner),
 		newMigration(316, "Add description for secrets and variables", v1_24.AddDescriptionForSecretsAndVariables),
+		newMigration(317, "Add new index for action for heatmap", v1_24.AddNewIndexForUserDashboard),
 	}
 	return preparedMigrations
 }