Use User#exempt_from_2sv? in user form fields partial

This method is more intention-revealing than calling `Object#blank?` on
the return value from `User#reason_for_2sv_exemption`.

app/views/users/_form_fields.html.erb

@@ -7,7 +7,7 @@
 </p>
 
 <% if policy(User).assign_role? %>
-  <% if @user.reason_for_2sv_exemption.blank? %>
+  <% if !@user.exempt_from_2sv? %>
     <p class="form-group">
       <%= f.label :role %><br />
       <%= f.select :role, options_for_select(assignable_user_roles.map(&:humanize).zip(assignable_user_roles), f.object.role), {}, class: "chosen-select form-control", 'data-module' => 'chosen' %>

test/integration/change_user_role_test.rb

@@ -13,7 +13,7 @@ def sign_in_as_and_edit_user(sign_in_as, user_to_edit)
   end
 
   context "when logged in as a super admin" do
-    should "be able to change the role of a user who has no 2SV exemption reason" do
+    should "be able to change the role of a user who is not exempt from 2SV" do
       user = create(:user)
       sign_in_as_and_edit_user(@super_admin, user)
 
@@ -23,7 +23,7 @@ def sign_in_as_and_edit_user(sign_in_as, user_to_edit)
       assert user.reload.admin?
     end
 
-    should "not be able to change the role of a user who has a 2SV exemption reason" do
+    should "not be able to change the role of a user who is exempt from 2SV" do
       user = create(:two_step_exempted_user)
       sign_in_as_and_edit_user(@super_admin, user)