Add tax_references, its controller and some basic pages for viewing ref data about taxes.

app/models/tax_reference.rb

@@ -0,0 +1,17 @@
+class TaxReference < ApplicationRecord
+  validates :year, uniqueness: true
+  MAX_MONTHLY_HOURS = 172
+
+  def worker_contribution(number_of_hours)
+    return self.hourly_wage * number_of_hours * self.worker_percentage / 100
+  end
+
+  def employer_contribution(number_of_hours)
+    return self.hourly_wage * number_of_hours * self.employer_percentage / 100
+  end
+
+  def total_contribution(number_of_hours)
+    return self.employer_contribution(number_of_hours) +
+        self.worker_contribution(number_of_hours)
+  end
+end