Add EveryPay payments

* Refactor BankLink into Payments::BankLink, add Payments::EveryPay
* Write tests for existing invoice views
* Write basic tests for Payments module

app/models/payments/base.rb

@@ -0,0 +1,49 @@
+module Payments
+  class Base
+    include ActionView::Helpers::NumberHelper
+
+    attr_reader :type,
+                :invoice,
+                :return_url,
+                :response_url,
+                :response
+
+    def initialize(type, invoice, opts = {})
+      @type         = type
+      @invoice      = invoice
+      @return_url   = opts[:return_url]
+      @response_url = opts[:response_url]
+      @response     = opts[:response]
+    end
+
+    def create_transaction
+      transaction = BankTransaction.where(description: invoice.order).first_or_initialize(
+        reference_no: invoice.reference_no,
+        currency: invoice.currency,
+        iban: invoice.seller_iban
+      )
+
+      transaction.save!
+    end
+
+    def complete_transaction
+      fail NotImplementedError
+    end
+
+    def settled_payment?
+      fail NotImplementedError
+    end
+
+    def form_fields
+      fail NotImplementedError
+    end
+
+    def form_url
+      ENV["#{type}_payment_url"]
+    end
+
+    def valid_response?
+      fail NotImplementedError
+    end
+  end
+end