Add spamicide field as a node form pane

@awolfey: Thanks for the patch. Please don't forget to update the issue status to "Needs review" when you do add a patch, that way the maintainer and other contributors can see that you've added something to be reviewed.

Looks good! I moved the preprocessor to the main module file, however, and changed the labels a bit to match the other node form panes, but otherwise it works as expected! I also added a node form pane for the actual "Add protection to this form" link as well for good measure.

Thanks!