Remove cluster association code duplication

What does this MR do?

Follow-up MR for comment: gitlab-foss!32559 (comment 215257593)

@DylanGriffith commented:

This MR is introducing several more instances of the application_#{application_name} thing. It seems to also introduce duplication of self.to_s.demodulize.underscore where there are many hardcoded examples already and now a dynamic method creating this thing. So perhaps it makes sense to actually introduce a method on ApplicationCore like:

def self.application_name
  self.to_s.demodulize.underscore
end

def self.association_name
  :"application_#{association_name}"
end

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