Buttons in security modalbox are broken after BS4 fix

Buttons in security modalbox are broken. Due to: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/stylesheets/pages/merge_requests.scss#L48

This was added to fix a BS4 regresion: https://gitlab.com/gitlab-org/gitlab-ce/commit/59b91de52de1e2cce416825887ec2f9ca8fe019b

@ClemMakesApps can you please take a look at this? Thanks!