Leverage Rails' caching when checking for admin

The privileges of a user should never change within a request
life cycle. The `spree_roles` association is very small, between 0 and 2
items are quickly searched in memory without the need of additional
database queries.

From memory, I've seen a lot of spree_roles queries in log files per
request. This should reduce it to one.

app/models/spree/user.rb

@@ -60,7 +60,7 @@ module Spree
 
     # Checks whether the specified user is a superadmin, with full control of the instance
     def admin?
-      spree_roles.where(name: "admin").any?
+      spree_roles.any? { |role| role.name == "admin" }
     end
 
     # Send devise-based user emails asyncronously via ActiveJob

spec/controllers/admin/order_cycles_controller_spec.rb

@@ -148,7 +148,6 @@ module Admin
                   order_cycles: 6,
                   proxy_orders: 1,
                   schedules: 1,
-                  spree_roles: 9,
                   spree_variants: 8,
                   tags: 1
                 },

spec/lib/reports/orders_and_distributors_report_spec.rb

@@ -140,8 +140,6 @@ RSpec.describe Reporting::Reports::OrdersAndDistributors::Base do
         subject # build context first
 
         expect { subject.table_rows }.to query_database [
-          "Spree::Role Exists?",
-          "Spree::Role Exists?",
           "SQL",
           "Spree::LineItem Load",
           "Spree::PaymentMethod Load",