Why and how to enable this is covered in the docs being changed
here. This requires gitlab-org/gitlab-shell@2e8b670 ("Add support for
SSH certificate authentication", 2018-06-14) which has been merged in
and tagged as 8.0.0, so GITLAB_SHELL_VERSION needs to be bumped.

Merging this closes gitlab-org/gitlab-ce#34572 see
gitlab-org/gitlab-shell!207 for the gitlab-shell MR.

Implementation notes:

 - The APIs being changed here are all internal, and their sole
   consumer is gitlab-shell.

 - Most of the changed code is a MR to gitlab-shell, see the
   gitlab-org/gitlab-shell!207 MR. That change covers why only some of
   the internal methods get a new "username" parameter, and why some
   others only get a "user_id".

Add support for searching users by confirmed e-mails

See merge request gitlab-org/gitlab-ce!20893

Resolve "Merge Request API response is missing changes_count"

Closes #47728

See merge request gitlab-org/gitlab-ce!19745

This is in preparation for modifying importers to assign e-mails for
only confirmed emails.

Fix Karma failures due to invalid icon names

See merge request gitlab-org/gitlab-ce!20887

The icon JSON field has been renamed from icon_status_success to status_success.

Resolve "Red master: can't modify frozen String in app/views/projects/jobs/_sidebar.html.haml"

Closes #49681

See merge request gitlab-org/gitlab-ce!20869

[ci skip]

Update gitlab-shell version to 7.2.0

See merge request gitlab-org/gitlab-ce!20876

Put fallback reply-key address first in the References header

See merge request gitlab-org/gitlab-ce!20871

Stop building Route rows on the fly

See merge request gitlab-org/gitlab-ce!20313

The References header is an ordered list of mails, and some clients
only look at the last mail in the list to determine to which mail the
mail is in reply to.

Therefore, to be a less obtrusive as possible, put the fallback
address first.

Some info about the reply key lookup in the References header:
https://docs.gitlab.com/ee/administration/reply_by_email.html#how-it-works

This reverts merge request !20679

Resolve "Making instance-wide data tools more accessible"

Closes #41416 and #48507

See merge request gitlab-org/gitlab-ce!20679

CE: Add PolicyCheckable concern for things passing to policy check

See merge request gitlab-org/gitlab-ce!20839

if all methods are also presented in the user.

It's sad that sanitizing is updating the string directly.

Resolve "The comparison list for the modified icons"

Closes #48567

See merge request gitlab-org/gitlab-ce!20501

More Gitaly cleanup: fetch_ref, allow disk access blocks

Closes gitaly#1295

See merge request gitlab-org/gitlab-ce!20802

Rails5 update Gemfile.rails5.lock

See merge request gitlab-org/gitlab-ce!20858

[master] Escapes milestone title shown in flash message when promoting a milestone

See merge request gitlab/gitlabhq!2436

[master] Fixed XSS in branch name in Web IDE

See merge request gitlab/gitlabhq!2431

[master] Escapes job name used in tooltips

See merge request gitlab/gitlabhq!2427

[master] Missing CSRF in System Hooks

See merge request gitlab/gitlabhq!2419

Fix namespace move callback behavior, especially to fix Geo replication of namespace moves during certain exceptions

Closes gitlab-ee#6252

See merge request gitlab-org/gitlab-ce!19297

Fix namespace move callback behavior, especially to fix Geo replication of namespace moves during certain exceptions

Resolve "BS4: Link in code brackets has two different styles"

Closes #48542

See merge request gitlab-org/gitlab-ce!20841