Rename reviewer related functions + allow for email matching instead of reviewer id

[emitanaka]

I often forget what the function names were for reviewer's who accept/reject reviewing (agree_reviewer() and decline_reviewer(), respectively, -- the words "agree" and "decline" just doesn't align with my mental mode). Since these are all reviewer related functions, would it be better to actually rename it as reviewer_agree(), reviewer_decline(), reviewer_abandon(), reviewer_add()?

Also could we allow reviewer identification to be email as an option instead of id? I'm bad at counting.