Fix memory and performance issues in Karma test suite

Closes #45900 and #45806

See merge request gitlab-org/gitlab-ce!18658

Bump nokogiri to 1.8.2 in qa Gemfile

See merge request gitlab-org/gitlab-ce!18673

Use gitaly 0.96.2

See merge request gitlab-org/gitlab-ce!18670

Fix commit trailer rendering when Gravatar is disabled

Closes #45633

See merge request gitlab-org/gitlab-ce!18586

Resolves CVE-2017-18258

Use gitlab-workhorse 4.2.0

See merge request gitlab-org/gitlab-ce!18671

Use persisted diff data instead fetching Git on discussions

Closes #44052

See merge request gitlab-org/gitlab-ce!18657

Add alpine mirrors while operating on k8s cluster

Closes gitlab-ee#4922

See merge request gitlab-org/gitlab-ce!18651

Merge request and commit discussions API

Closes #20901

See merge request gitlab-org/gitlab-ce!18537

Use Ruby methods, not Rails' String#first

See merge request gitlab-org/gitlab-ce!18669

[CE] Exclude LDAP from OmniauthCallbackController base methods

See merge request gitlab-org/gitlab-ce!18619

show only groups an admin is a member of in dashboards/grops

See merge request gitlab-org/gitlab-ce!17884

Gitaly-Ruby is not a Rails application, which made the method not exist
after the vendor. This patch uses direct indexing, which has the same
properties that `#first` has.

Found while working on: https://gitlab.com/gitlab-org/gitaly/merge_requests/699