Reviewer: add missing constraint
Reviewer: add missing constraint

add unique constraint on pull_request_reviewer's pull_request_id and user_id

oolso/yobi,doortts/fork-yobi,ahb0327/yobi,yona-projects/yona,doortts/forked-for-history,doortts/fork-yobi,Limseunghwan/oss,Limseunghwan/oss,ihoneymon/yobi,naver/yobi,oolso/yobi,yona-projects/yona,naver/yobi,bloodybear/yona,ahb0327/yobi,brainagenet/yobi,ihoneymon/yobi,ahb0327/yobi,bloodybear/yona,naver/yobi,doortts/fork-yobi,oolso/yobi,brainagenet/yobi,bloodybear/yona,doortts/fork-yobi,yona-projects/yona,doortts/forked-for-history,Limseunghwan/oss,brainagenet/yobi,bloodybear/yona,yona-projects/yona,doortts/forked-for-history,ihoneymon/yobi