CLDC-1878 Add hint to mortgage amount question

app/models/form/sales/questions/mortgage_amount.rb

@@ -8,5 +8,6 @@ class Form::Sales::Questions::MortgageAmount < ::Form::Question
     @min = 0
     @width = 5
     @prefix = "£"
+    @hint_text = "Enter the amount of mortgage agreed with the mortgage lender. Exclude any deposits or cash payments. Numeric in pounds. Rounded to the nearest pound."
   end
 end

spec/models/form/sales/questions/mortgage_amount_spec.rb

@@ -32,7 +32,7 @@ RSpec.describe Form::Sales::Questions::MortgageAmount, type: :model do
   end
 
   it "has the correct hint" do
-    expect(question.hint_text).to be_nil
+    expect(question.hint_text).to eq("Enter the amount of mortgage agreed with the mortgage lender. Exclude any deposits or cash payments. Numeric in pounds. Rounded to the nearest pound.")
   end
 
   it "has correct width" do