Don't add author and reviewers to CCs

Summary:
Commenting on a diff causes adding the writer to the CCs. It doesn't make much
sense if the writer is author or reviewer who get all the copies anyway.

I've also moved the decision to DifferentialCommentEditor.

Test Plan:
Comment on a diff where I am author
Comment on a diff where I am reviewer
Comment on a diff where I am neither
Explicitely Add CCs where I am author

Reviewers: epriestley

Reviewed By: epriestley

CC: jungejason, aran, epriestley

Differential Revision: https://secure.phabricator.com/D1397