Nest LenderAdmin and LenderUser resources under Lender

CfeAdmins manage LenderAdmins across multiple Lenders. Access to LenderAdmins in this case is now via the Lender index.
DRY up request spec LenderAdmin management navigation
Show correct breadcrumbs on lender admin user management pages
Refactor UserHelper
Make LenderAdminsController#lender private
Use size instead of count for number of lenders for a user
Use path helpers in tests for polymorphic_user_path
Tweak polymorphic_user_path interface

Switched ordering to make the first argument optional. Order now matches polymorphic_path.
Merge pull request #133 from alphagov/filter_users_by_lender

CfE Admins access lender admins grouped by lender