Pull Request: C5_Khaled_450_Contest_Timer_5.28

Issue #450 
Reviewers: @Husseny @linsara @nadineadel @MussabElDash 

<b>For Scenario Reviewing:</b>
1. Open Rails console using ```rails c```
2. In the console, type the following:
```
c = Contest.first
c.start_time = TO_BE_REPLACED_1
c.end_time = TO_BE_REPLACED_2
c.save
```
3. In the browser, navigate to ```/contests/1```

Examples for TO_BE_REPLACED_1 & TO_BE_REPLACED_1:
```DateTime.now + 10.minutes```
```DateTime.now - 5.minutes```
```DateTime.now + 2.hours```
and so on.

So you should try different scenarios for the start_time and end_time then check how the timer will respond.