Add the decoupling metrics rake task

To get the metrics, run the following rake task:
`rake metrics:decoupling`

lib/tasks/metrics.rake

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+namespace :metrics do
+  desc 'Prints out some basic decoupling metrics'
+  task decoupling: :environment do
+    erb_nr = 0
+    jsx_nr = 0
+    Dir.glob('**/*.erb') { erb_nr += 1 }
+    Dir.glob('**/*.jsx') { jsx_nr += 1 }
+
+    # Print the metrics
+    puts ''
+    puts '**********************************'
+    puts '**                              **'
+    puts '**     Decoupling metrics       **'
+    puts '**                              **'
+    puts '**********************************'
+    puts ''
+    puts 'Nr. of ERB template files (*.erb):'
+    puts erb_nr
+    puts ''
+    puts 'Nr. of React template files (*.jsx):'
+    puts jsx_nr
+    puts ''
+    puts '**********************************'
+    puts ''
+  end
+end