tasks: Add mass actions

include/ajax.tasks.php

include/ajax.tickets.php

include/class.dynamic_forms.php

include/class.forms.php

include/class.task.php

include/class.ticket.php

include/staff/templates/dynamic-form.tmpl.php

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

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

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

include/staff/templates/task.tmpl.php

include/staff/templates/tasks-actions.tmpl.php

include/staff/tickets.inc.php