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.
Even if something is a nit, I think I'd for the most part like the contributor to take care of it. Perhaps it would be good to add, "Authors should try their hardest to address these nits so that the code maintains high quality" or something. I don't want people thinking that they only have to address the Must comments. Why would I bother making nits if you can blanket ignore them?