Reduce the number of Elasticsearch client instances that are created

Closes #3650

See merge request gitlab-org/gitlab-ee!3432

Add allow remote mirrors to be setup for projects documentation.

See merge request gitlab-org/gitlab-ee!3425

CE Port: Performance issues when loading large number of wiki pages

See merge request gitlab-org/gitlab-ee!3420

When collecting instance profile credentials in AWS, each client instantiation
is a HTTP request to an external web service. This service may rate-limit us
if we perform too many requests in a given time period.

A typical GitLab deployment will have many processes running, and each of those
processes needs *one* elasticsearch client instance. The client instance is
thread-safe and handles concurrent requests very well, with HTTP keep-alive
connections.

Prior to this commit, each process using elasticsearch would instantiate one
client per *class* that used `Elasticsearch::Model::Client`. So a multi-node
setup might look like:

```
* Server A
  * Unicorn parent
    * Unicorn child A
      * Client for Project class
      * Client for Repository class
      * Client for Issue class
      * ...
    * Unicorn child B
      * Client for Project class
      * Client for Repository class
      * Client for Issue class
      * ...
  * Sidekiq
    * Client for Project class
    * Client for Repository class
    * Client for Issue class
    * ...
* Server B
  * Unicorn master
    * ... (same as above)
  * Sidekiq
    * .... (same as above)
```

(total: N, plus N per unicorn child, multipled by the number of servers)

Following this commit, we have the following clients instead:

```
* Server A
  * Sidekiq (1 client)
  * Unicorn parent
    * Unicorn child A (1 client)
    * Unicorn child b (1 client)
* Server B
  * ... (same as above)
```

(total: 1, + 1 per unicorn child, multipled by the number of servers)

This drastically reduces the number of HTTP connections we make to the
Elasticsearch and AWS instance profile credentials servers, and should
come with a small increase in performance due to better utilisation of
those connections.

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Closes #3775

See merge request gitlab-org/gitlab-ee!3207

Port to EE: Set 0.69.0 instead of latest for codeclimate image

See merge request gitlab-org/gitlab-ee!3446

Exports a couple of project related code as es6 modules

See merge request gitlab-org/gitlab-ee!3374

Improve geo docs post demo

See merge request gitlab-org/gitlab-ee!3436

Set 0.69.0 instead of latest for codeclimate image

See merge request gitlab-org/gitlab-ce!15445

Fix: Geo API bug. Statistic is not collected when prometheus is disabled

Closes #4055

See merge request gitlab-org/gitlab-ee!3421

Docs: add omnibus roles for external PostgreSQL

Closes #2825

See merge request gitlab-org/gitlab-ee!3286

Slack application landing page

Closes #3027

See merge request gitlab-org/gitlab-ee!2415

The Geo log cursor should only run on secondary nodes

Closes #4049

See merge request gitlab-org/gitlab-ee!3411

Revert "Update nfs.md with information on AWS EFS and Burst Credit usage and how to…"

See merge request gitlab-org/gitlab-ee!3385

Remove duplicate delete button in epic

Closes #4064

See merge request gitlab-org/gitlab-ee!3429

This reverts commit fa1ff54f

Geo documentation fixes following on from 2017-11-14 demo

Closes #4052

See merge request gitlab-org/gitlab-ee!3410

Merge branch 'rc/use-application_setting-current-in-admin-application_settings_controller' into 'master'

Use ApplicationSetting.current in Admin::ApplicationSettingsController

See merge request gitlab-org/gitlab-ee!3323

Upgrade gitlab-svgs to 1.1.0

Closes #4047

See merge request gitlab-org/gitlab-ee!3414

Correct the host used for consul->pgbouncer pgpass

Closes gitlab-org/build/team-tasks#45

See merge request gitlab-org/gitlab-ee!3426

Introduce a new EE::Admin::UsersController module

See merge request gitlab-org/gitlab-ee!3319

Introduce a new EE::Concerns::LfsRequest module

See merge request gitlab-org/gitlab-ee!3322

Tweak consul.configuration.retry_join examples

Closes gitlab-org/build/team-tasks#29

See merge request gitlab-org/gitlab-ee!3422

Introduce a new EE::ServiceParams module

See merge request gitlab-org/gitlab-ee!3321

Introduce a new EE::Admin::GroupsController module

See merge request gitlab-org/gitlab-ee!3320

On failover the consul agent notifies the pgbouncer by running
`gitlab-ctl pgb-notify --newhost ADDRESS --user pgbouncer --hostuser gitlab-consul`.

For the documented configuration, this turns into a connection to the
pgbouncer service on 127.0.0.1, which may not be what was selected for
PGBOUNCER_HOST in the pgpass file, leading to authentication failures.

In user information we introduce a new variable, CONSUL_SERVER_NODES,
to name this existing 'You need to know the addresses of the consul
servers' piece of information.  Then later in the example configuration
files we refer back to that variable.

https://gitlab.com/gitlab-org/build/team-tasks/issues/29

Add warning when Geo is configured insecurely

Closes #3904

See merge request gitlab-org/gitlab-ee!3368