pundit: convert user name change requests to pundit.

Fix discrepancy between index action and show action. The index
action allowed members to see name changes for undeleted users, but the
show action didn't.

app/policies/user_name_change_request_policy.rb

@@ -0,0 +1,13 @@
+class UserNameChangeRequestPolicy < ApplicationPolicy
+  def index?
+    user.is_member?
+  end
+
+  def show?
+    user.is_admin? || (user.is_member? && !record.user.is_deleted?) || (record.user == user)
+  end
+
+  def permitted_attributes
+    [:desired_name, :desired_name_confirmation]
+  end
+end