Add warning to w/b lists

[dmorton]

w/b lists tend to get abused, as everyone thinks they should put all their friends in a whitelist. This robs Maia of the ability to learn the difference and degrades its effectiveness. It also allows for the possibility of forged spam to make it through the filters.

Perhaps some sort of help text on the wblist screen would help educate people.... well, it's a dream...