Drupal views: Change handler for activity details field to civicrm_handler_field_markup

Presently any html markup in an acitivity details field will be output into a view with the html markup escaped and rendered as html entities.

This patch switches the handler for this field to from views_handler_field to civicrm_handler_field_markup. Coupled with my work on the markup filter (http://issues.civicrm.org/jira/browse/CRM-10496) this will successfully allow html markup to be displayed properly.

This issue has previously been raised in the forums, eg. http://forum.civicrm.org/index.php/topic,25067.msg105871.html#msg105871 and http://forum.civicrm.org/index.php/topic,10259.0.html