Popular courses(NEW), dynamic images for courses(NEW), disenroll course(FIX)

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controllers:

-	DB.java: new methodes(getPopularCourses, amountPlusOne, amountMinusOne), changed(getCourse, getCourses, searchCourse)

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models:

-	Course added get/set ImgSrc for dynamic course images

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controllers:

-	popularCourses(NEW)
-	courseCatalog
-	disenrollCourse(FIX)
-	enrollCourse

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pages:

-	popularCourses(NEW)
-	courseCatalog
-	courseDetails
-	myCourses
-	searchCourse

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