Remove manual Vendor prefixes in CSS

Summary

We have a number of manual vendor prefixes in the codebase:

~/workspace/gdk-ee/gitlab master
❯ git grep -E '\-webkit\-|\-moz\-|\-mz\-o\-' | wc -l
      75

Notably files like: https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fassets%2Fstylesheets%2Ftest.scss

Manually adding prefixes is error prone and easy to forget or miss during reviews. Autoprefixes should be enabled for these cases.

Improvements

Ensure auto-prefixer works for these instances and remove manual vendor prefixes.

Risks

Automatic vendor prefixes might not work for the listed cases; Ensure we test on relevant browsers.

Involved components

app/assets/javascripts/lib/utils/sticky.js
app/assets/stylesheets/bootstrap_migration.scss
app/assets/stylesheets/emoji_sprites.scss
app/assets/stylesheets/framework/animations.scss
app/assets/stylesheets/framework/dropdowns.scss
app/assets/stylesheets/framework/flash.scss
app/assets/stylesheets/framework/forms.scss
app/assets/stylesheets/framework/mixins.scss
app/assets/stylesheets/framework/selects.scss
app/assets/stylesheets/framework/sortable.scss
app/assets/stylesheets/framework/typography.scss
app/assets/stylesheets/mailer_client_specific.scss
app/assets/stylesheets/pages/builds.scss
app/assets/stylesheets/pages/diff.scss
app/assets/stylesheets/pages/issuable.scss
app/assets/stylesheets/pages/merge_requests.scss
app/assets/stylesheets/pages/search.scss
app/assets/stylesheets/pages/wiki.scss
app/assets/stylesheets/test.scss
ee/app/assets/stylesheets/components/feature_flags/form.scss
ee/app/assets/stylesheets/pages/roadmap.scss
ee/app/views/notify/approved_merge_request_email.html.haml
ee/app/views/notify/unapproved_merge_request_email.html.haml
lib/tasks/gemojione.rake
scripts/frontend/stylelint/stylelint-utils.js
spec/fixtures/emails/on_wrote.eml
vendor/assets/stylesheets/cropper.css