Fixes group/instance level cluster feature flag checking.

Fixes issues with show whitespace button in diffs

Closes #42597 and #58852

See merge request gitlab-org/gitlab-ce!28242

Add Let's Encrypt client

See merge request gitlab-org/gitlab-ce!27624

Part of adding Let's Encrypt certificates for pages domains

Add acme-client gem

Client is being initialized by private key stored in secrets.yml
Let's Encrypt account is being created lazily.
If it's already created, Acme::Client just gets account_kid by
calling new_account method

Make Let's Encrypt client an instance
Wrap order and challenge classes

Properly clear the merge error upon rebase failure

Closes #56139

See merge request gitlab-org/gitlab-ce!28319

i18n JS files starting with i

See merge request gitlab-org/gitlab-ce!28010

Resolve CE/EE diffs in help/index

Closes gitlab-ee#11051

See merge request gitlab-org/gitlab-ce!28335

Resolve CE/EE diffs in signup_box

Closes gitlab-ee#11047

See merge request gitlab-org/gitlab-ce!28336

If `merge_error` is ever set after a failure, it is never cleared, even
after a subsequent, successful rebase. We now clear this field whenever
a successful rebase occurs.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56139

clarify that "team" means "project" when considered for assignment purposes.

See merge request gitlab-org/gitlab-ce!27753