Common terminology for review comments

@singingwolfboy @sarina @nedbat Please take a look at the suggested terminology and definition proposed in this PR.  I spoke to @singingwolfboy about the benefits of having a common vocabulary for this and he proposed creating this PR to discuss it.  I'd appreciate your feedback.  I find this will be useful within our T&L team and possibly for edX at large.
@cahrens FYI.
@nasthagiri I don't think that Github supports anything like that, and frankly, I doubt that it will in the future, either.