Fix Issuable#add_labels_by_names to validate if label exists on group
Fix Issuable#add_labels_by_names to validate if label exists on group

axilleas/gitlabhq,dreampet/gitlab,axilleas/gitlabhq,htve/GitlabForChinese,t-zuehlsdorff/gitlabhq,htve/GitlabForChinese,dplarson/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,dreampet/gitlab,dplarson/gitlabhq,openwide-java/gitlabhq,LUMC/gitlabhq,openwide-java/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,dreampet/gitlab,daiyu/gitlab-zh,darkrasid/gitlabhq,jirutka/gitlabhq,LUMC/gitlabhq,openwide-java/gitlabhq,stoplightio/gitlabhq,darkrasid/gitlabhq,stoplightio/gitlabhq,jirutka/gitlabhq,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,daiyu/gitlab-zh,LUMC/gitlabhq,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,iiet/iiet-git,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,dplarson/gitlabhq,openwide-java/gitlabhq,LUMC/gitlabhq,dreampet/gitlab,daiyu/gitlab-zh,htve/GitlabForChinese,jirutka/gitlabhq,htve/GitlabForChinese,darkrasid/gitlabhq,daiyu/gitlab-zh,iiet/iiet-git,darkrasid/gitlabhq,iiet/iiet-git,jirutka/gitlabhq,mmkassem/gitlabhq