Job Posts: Developer interest in job posts should be counted towards the platform-wide response rate.

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Parent issue: #30

I see the current response rate is the number of developers who responded to an employer-initiated message:

talent.tampa.dev/app/models/pricing/stats.rb

Lines 7 to 9 in b3f552b

	def response_rate
	responded_to_conversations.fdiv(conversations)
	end

I gather the job post response rate would be the number of job posts that get at least one application.

Are we wanting to combine these two metrics into one? I think it makes sense to keep them separate, so on the website we can say something like 87% of job postings get at least one applicant in a clean way.

So we'd have e.g. conversations_response_rate and job_response_rate.