Revert unintentional changes in !12810

Due to repeated rebases on it through almost 4 months, there are some unintentional changes to be removed in !12810 (merged) as follows:

app/assets/stylesheets/framework/header.scss
app/assets/stylesheets/framework/secondary-navigation-elements.scss
app/assets/stylesheets/framework/selects.scss
app/assets/stylesheets/pages/environments.scss
app/assets/stylesheets/pages/issuable.scss
app/assets/stylesheets/pages/merge_requests.scss
app/assets/stylesheets/pages/note_form.scss
app/assets/stylesheets/pages/notes.scss
app/assets/stylesheets/pages/search.scss