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

Resolve "Add new Unicorn metrics"

Closes #56850

See merge request gitlab-org/gitlab-ce!27474

Remove puts

Remove puts

Added Omniauth OpenID Connect strategy

See merge request gitlab-org/gitlab-ce!27383

This also supports Ruby 2.6.

This adds Ruby 2.6 support.

Switch to sassc-rails for faster stylesheet compilation

Closes #18432

See merge request gitlab-org/gitlab-ce!26224

The new two-step Gitaly `Rebase` RPC yields the rebase commit SHA to the
client before proceeding with the rebase.

This avoids an issue where the rebase commit SHA was returned when the
RPC had fully completed, and in some cases this would be after the Rails
`post_receive` worker services had already run. In these situations,
the merge request did not yet have its rebase_commit_sha attribute set
introducing the possibility for bugs (such as previous approvals being
reset).

https://gitlab.com/gitlab-org/gitlab-ee/issues/5966

Extends existing graphql's tracer with opentracing measurements. Because
it also adds Tracing::Graphql class (for opentracing), it also renames
Graphql::Tracing class to Graphql::GenericTracing to minimize confusion
with similar class names.