Refinements to the new application member modal

- remove extra padding from second modal screen
- update modal styles to more closely match designs
- ad `user_name` property to application_role model to fix flash message

templates/fragments/applications/add_new_application_member.html

@@ -41,7 +41,7 @@
   </div>
 {% endset %}
 {% set step_two %}
-  <div class="modal__form--padded">
+  <div class="modal__form">
     <div class="modal__form--header">
       <h1>{{ "portfolios.applications.members.new.assign_roles" | translate }}</h1>
       <a class='icon-link'>