templates: menu for About CMS Physics Objects

* Adds menu for the About CMS Physics Objects page.
  (closes #417) (PR #502)
templates: fixes FIXME on CMS Physics Objects

* Replaces FIXME with correct links on the About CMS Physics Objects
  page.  (closes #296) (PR #502)
templates: EGamma link on CMS Physics Objects page

* Fixes incorrect link to EGamma TWiki on About CMS Physics Objects
  page.  (PR #502)