Remove ApplicationRecord

This gem redefines the host apps ApplicationRecord class,
which is very wrong!

We introduce a namespaced BaseRecord and set the correct
table_name_prefix on that class.

app/models/solidus_braintree/base_record.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+module SolidusBraintree
+  class BaseRecord < ::Spree::Base
+    self.abstract_class = true
+    self.table_name_prefix = 'solidus_paypal_braintree_'
+  end
+end

app/models/solidus_braintree/configuration.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module SolidusBraintree
-  class Configuration < ::Spree::Base
+  class Configuration < BaseRecord
     self.table_name = "solidus_paypal_braintree_configurations"
 
     PAYPAL_BUTTON_PREFERENCES = {

app/models/solidus_braintree/customer.rb

@@ -1,9 +1,7 @@
 # frozen_string_literal: true
 
 module SolidusBraintree
-  class Customer < ApplicationRecord
-    self.table_name = "solidus_paypal_braintree_customers"
-
+  class Customer < BaseRecord
     belongs_to :user, class_name: ::Spree::UserClassHandle.new, optional: true
     has_many :sources, class_name: "SolidusBraintree::Source", inverse_of: :customer, dependent: :destroy
   end