Reuse common code from leaderboard API

As in [this](https://github.com/Cloud-CV/EvalAI/pull/2798) PR we have created an API for not only giving public submissions but also private submissions to display on the leaderboard. So we have used most the things same which are present in the [leaderboard API](https://github.com/Cloud-CV/EvalAI/blob/master/apps/jobs/views.py#L467).

**How to fix this issue:**

In order to fix this issue one need to move that reusable code from both the APIs to some common place and then use it from there in both the APIs.