Bootstrap 4.1.2 upgrade blocked
Upgrading bootstrap breaks vertical alignment of lots of our icons.
From https://gitlab.com/gitlab-org/gitlab-ce/issues/51362
4.1.2 contains changes in the svg selector for reboot that added vertical align middle (https://github.com/twbs/bootstrap/blob/v4.1.2/scss/_reboot.scss#L275-L278)