apply bst4 button

src/client/js/components/Admin/UserManagement.jsx

@@ -55,7 +55,7 @@ class UserManagement extends React.Component {
         {adminUsersContainer.state.userForPasswordResetModal && <PasswordResetModal />}
         <p>
           <InviteUserControl />
-          <a className="btn btn-default btn-outline ml-2" href="/admin/users/external-accounts">
+          <a className="btn page-link text-dark d-inline-block ml-2" href="/admin/users/external-accounts" role="button">
             <i className="icon-user-follow" aria-hidden="true"></i>
             { t('user_management.external_account') }
           </a>

src/client/js/components/Admin/Users/InviteUserControl.jsx

@@ -14,7 +14,7 @@ class InviteUserControl extends React.Component {
 
     return (
       <Fragment>
-        <button type="button" className="btn btn-default" onClick={adminUsersContainer.toggleUserInviteModal}>
+        <button type="button" className="btn page-link text-dark d-inline-block" onClick={adminUsersContainer.toggleUserInviteModal}>
           { t('user_management.invite_users') }
         </button>
         <UserInviteModal />