Remove instance variable assignment from `gitlab_sign_in` helper
Remove instance variable assignment from `gitlab_sign_in` helper

Also removes `Thread.current[:current_user]` assignment from
`gitlab_sign_in_via`

dreampet/gitlab,stoplightio/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,iiet/iiet-git,t-zuehlsdorff/gitlabhq,t-zuehlsdorff/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,dreampet/gitlab,axilleas/gitlabhq,dplarson/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,dreampet/gitlab,stoplightio/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,dplarson/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,iiet/iiet-git,dplarson/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,t-zuehlsdorff/gitlabhq,iiet/iiet-git