Allow access to /admin by default when user has any custom admin role

Implementation plan

• Allow access to /admin by default when user has any custom admin role (implemented in !189148 (merged))
• Clean up unnecessary redirect logic introduced for custom admin roles (implemented in !189148 (merged))
  • ee/app/helpers/ee/sidebars_helper.rb
• Remove the existence of read_admin_dashboard permission (implemented in !189132 (merged))