Fix sending a notification to mentioned users.
Fix sending a notification to mentioned users.

The feature was removed by 5e47389 accidentally.

ChangsungKim/TestRepository01,doortts/fork-yobi,naver/yobi,naver/yobi,brainagenet/yobi,doortts/fork-yobi,bloodybear/yona,ChangsungKim/TestRepository01,ihoneymon/yobi,yona-projects/yona,doortts/forked-for-history,bloodybear/yona,oolso/yobi,ahb0327/yobi,brainagenet/yobi,bloodybear/yona,oolso/yobi,bloodybear/yona,violetag/demo,Limseunghwan/oss,ahb0327/yobi,naver/yobi,ahb0327/yobi,violetag/demo,Limseunghwan/oss,ihoneymon/yobi,yona-projects/yona,yona-projects/yona,doortts/forked-for-history,yona-projects/yona,brainagenet/yobi,ihoneymon/yobi,oolso/yobi,doortts/fork-yobi,doortts/fork-yobi,Limseunghwan/oss,doortts/forked-for-history