Add collaboration support to task

include/ajax.tasks.php

include/ajax.thread.php

include/ajax.tickets.php

include/class.collaborator.php

include/class.task.php

include/class.thread.php

include/class.ticket.php

include/staff/tasks.inc.php

include/staff/templates/collaborators-preview.tmpl.php

include/staff/templates/collaborators.tmpl.php

include/staff/templates/task-preview.tmpl.php

include/staff/templates/task-view.tmpl.php

include/staff/templates/ticket-preview.tmpl.php

include/staff/ticket-view.inc.php

scp/ajax.php