add command to migrate Version.approvalnotes to comm (bug 1091160)

To be run after https://github.com/mozilla/zamboni/pull/2689

"When developers submit a new version of their app, they leave "Comments for Reviewers", which is Version.approvalnotes in the code. These "Comments for Reviewers" now makes more sense as a CommunicationNote rather than a field attached to the Version.

Adding it as a CommunicationNote allows the Comments for Reviewers to be visible within the Communication Dashboard."

Not stripping out Version.approvalnotes yet, a lot to do before then.