"whom" and "whomever" are only necessary when the pronoun is used as an object of a verb, and even for that people tend to prefer "who" and "whoever" nowadays.
ex) I thank whomever it was that created this. (object of "thank").
(Most people say "I thank whoever..." for this)
In your sentence, it only serves as a subject, so there is absolutely no reason why "whomever" has to enter into the picture. It should be "Whoever".