benefits: Display member button.

benefits: Display member button.

* CRM/Admin/Form/Setting/MemberDashboard.php
  (CRM_Admin_Form_Setting_MemberDashboard) [buildQuickForm]: Add input
  for static button URL.
  [postProcess]: Save static button URL.
* CRM/Memberdashboard/Page/Benefits.php
  (CRM_Memberdashboard_Page_Benefits) [run]: Add 'contact' and
  'buttonUrl' smarty variables.
* settings/MemberDashboard.setting.php
  (memberdashboard_button_static_url): New setting.
* templates/CRM/Admin/Form/Setting/MemberDashboard.tpl: Render static
  button URL form field.
* templates/CRM/Memberdashboard/Page/Benefits.tpl: Render member button.