***** NOT A TEACHER *****
I was astonished ("shocked," actually) to learn that Barbara, an American writer, seems to endorse the majority view in this thread.
I am pretty sure that most Americans would consider "who make this song so popular" as simply an adjective clause modifying "you." Of course, most would be a little rusty on their grammar, so they probably could not explain their choice, except to point out that "who" follows the word "you."
For example, I may be wrong (although I do not believe that I am) that most Americans would "naturally" say something like:
"Don't believe what other people are telling you, Tom. It is you who are my best friend!"
I doubt that most Americans would accept "It is you who is my best friend." (Yes, grammatically, it is correct. But today most Americans, for example, expect "It is me" over the "correct" "It is I.")
*****
I can only assume that many of the posters in this thread also disagree with this sentence in Descriptive English Grammar by House and Harman:
"It is I who am at fault." The authors explain that "the verb 'am' shows us that the relative pronoun agrees with its antecedent in person and number."
*****
I am very concerned that some learners who come to the United States and say "It is you who is my best friend" may be confused if some Americans (even some teachers) look at them askance.
James