Get living wage min age from rates file for start

Update the start page templates to dynamically fetch the minimum age to
 be eligible for the living wage from the rates file rather than being
 hard-coded in the template, similar to how is already done for the
 "last year end text".

app/flows/am_i_getting_minimum_wage_flow/start.erb

@@ -13,7 +13,7 @@
   - you’re getting paid the National Living Wage
   - your employer owes you past payments from the previous year because of underpayment
 
-  ^You must be at least 23 years old to get the National Living Wage.^
+  ^You must be at least <%= this_year_living_wage_min_age("minimum_wage") %> years old to get the National Living Wage.^
 
   Check [National Minimum Wage and National Living Wage rates](/national-minimum-wage-rates) before <%= last_year_end_text("minimum_wage") %>.
 <% end %>

app/flows/minimum_wage_calculator_employers_flow/start.erb

@@ -13,7 +13,7 @@
   - you’re paying a worker the National Living Wage
   - you owe your employee payments from the previous year because you underpaid them
 
-  ^Your employee must be at least 23 years old to get the National Living Wage.^
+  ^Your employee must be at least <%= this_year_living_wage_min_age("minimum_wage") %> years old to get the National Living Wage.^
 
 Check [National Minimum Wage and National Living Wage rates](/national-minimum-wage-rates) before <%= last_year_end_text("minimum_wage") %>.
 <% end %>

lib/smart_answer/rate_dates_helper.rb

@@ -8,6 +8,15 @@ def last_year_end_text(rates_file)
       last_year(rates_file)[:end_date].strftime("%B %Y").to_s
     end
 
+    def this_year_living_wage_min_age(rates_file)
+      this_year(rates_file)[:living_wage_min_age]
+    end
+
+    def this_year(rates_file)
+      data = SmartAnswer::Calculators::RatesQuery.from_file(rates_file)
+      data.current_period
+    end
+
     def last_year(rates_file)
       data = SmartAnswer::Calculators::RatesQuery.from_file(rates_file)
       data.previous_period