limit what can be mass edited

after looking into this problem further, i decided that the only sane solution at the moment is to limit the data that can be mass edited, as it's too difficult to predict what kinds of edits will cause errors/problems. i've adjusted the code so that mass edits can be performed on node title, body, teaser, and event start and end times. these should be the most likely things requiring editing, and are all stable things to edit.