Pull Request: C4_Lin_track_rating_4.3

<b>For scenario reviewers :- </b> 
1) Sign in as email = "1@lecturer.com"  , password = "123456789"
2) Navigate to "/topics/1"
3) Click on "Edit Track Rating" button and drag and drop the tracks to change their difficulty rating.

You can also test with any other TA/Lecturer, but make sure that you assign them the correct courses (i.e "1@lecturer.com" was assigned to course 1 which contains topic 1, therefore we were able to use the "/topics/1" navigation)

You can assign a course to the lecturer/ta by :- 
lec = Lecturer.last()
c = Course.first()
lec.courses << c
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#182 @rami-khalil @mimikian @Sharaf93 @fadel1493 @mohamedsaeed93