Track whether anonymous feedback is actionable

This is useful when wanting to filter out duplicates, spam and generally
non-actionable feedback.
Hide non-actionable feedback in feedex
Don't allow is_actionable to be null
AnonymousContact: change class methods to scopes
Merge pull request #143 from alphagov/hide-non-actionable-feedback-in-feedex

Track and hide non-actionable feedback in Feedex