Notify only about slow reports

It would be annoying to get an email for reports which display on the
screen immediately.

app/jobs/report_job.rb

@@ -2,11 +2,18 @@
 
 # Renders a report and stores it in a given blob.
 class ReportJob < ApplicationJob
+  NOTIFICATION_TIME = 5.seconds
+
   def perform(report_class, user, params, format, blob)
+    start_time = Time.zone.now
+
     report = report_class.new(user, params, render: true)
     result = report.render_as(format)
     blob.store(result)
-    email_result
+
+    execution_time = Time.zone.now - start_time
+
+    email_result if execution_time > NOTIFICATION_TIME
   end
 
   def email_result

spec/jobs/report_job_spec.rb

@@ -32,6 +32,9 @@ describe ReportJob do
     # Setup test data which also triggers emails:
     report_args
 
+    # Send emails for quick jobs as well:
+    stub_const("ReportJob::NOTIFICATION_TIME", 0)
+
     expect {
       # We need to create this job within the block because of a bug in
       # rspec-rails: https://github.com/rspec/rspec-rails/issues/2668
@@ -40,6 +43,18 @@ describe ReportJob do
     }.to enqueue_mail(ReportMailer, :report_ready)
   end
 
+  it "triggers no email when the report is done quickly" do
+    # Setup test data which also triggers emails:
+    report_args
+
+    expect {
+      # We need to create this job within the block because of a bug in
+      # rspec-rails: https://github.com/rspec/rspec-rails/issues/2668
+      ReportJob.perform_later(*report_args)
+      perform_enqueued_jobs(only: ReportJob)
+    }.to_not enqueue_mail
+  end
+
   def expect_csv_report
     blob.reload
     expect(blob.filename.to_s).to eq "report.csv"