Use Akismet on suggestions

[singerb]

We talked to Chris Finke about this. The salient points are:

• Mozilla is doing this on their MDN wiki
• pass wiki-revision as the type
• only pass the added lines from the edit
• reasonable to use this only to display some info on the approval screen, maybe discard very bad edits indicated by Akismet
• check the Jetpack contact form code for how to utilize the Akismet plugin to do most of the work for you (including API key)