Add reference to the README  to upgrade guide CE to EE

Issue - https://dev.gitlab.org/gitlab/gitlab-ee/issues/183

See merge request !213

WIP. CE stable to EE stable

```
Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   config/initializers/1_settings.rb
	both modified:   config/initializers/7_omniauth.rb
	both modified:   features/admin/groups.feature
	both modified:   features/steps/admin/groups.rb

```

@jacobvosmaer There are 4 conflicts. Two of them (last two ones) I merged by myself. In other cases your help would be appreciated.

See merge request !209

Error messages fix

See merge request !1201

[7.4 rc fix] [doc] clarify when database.yml needs to be updated

[7.4 rc fix] [doc] stop gitlab before mysql optimizations, run checks

[7.4 rc fix] [doc] actually give command necessary to update unicorn

[7.4 rc fix] [doc] stop gitlab before backup

[7.4 rc fix] [doc] add missing configure Redis to use sockets

[7.4 rc fix] [doc] add optimizations for mysql to 6.x->7.4 guide

Fix CE-EE LDAP merge conflicts

See merge request !210

Conflicts:
	config/initializers/1_settings.rb
	config/initializers/7_omniauth.rb
	features/admin/groups.feature
	features/steps/admin/groups.rb

Fixes from master

It includes:

* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1193
* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1196
* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1195

See merge request !1198

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

We need to skip empty repositories when creating a backup. Before this
change, we were asking gollum-lib if the wiki contains any _pages_. Now
we ask gitlab_git if the repository contains _files_. This should
resolve gollum_lib Grit timeouts in the backup script.

Add mysql optimizations from 7.3-to-7.4.md.

Add details from 7.2-to-7.3.md. Replaces https://github.com/gitlabhq/gitlabhq/pull/8047.

Stopping gitlab before backup ensures that backup has everything before upgrade incase something goes wrong. Also remove extra cd.

Give command to update unicorn.rb rather than just say you need to do it.

Update MySQL optimizations to reflect doing updates after GitLab has already been started back up.

Clarify that database.yml should be updated if you are running a MySQL Database. Remove wording that de-emphasises importance of update.

7-4-stable CE to EE

See merge request !207

Conflicts:
	VERSION
	doc/install/installation.md

Added EE changelog

See merge request !204

CE master to EE master

See merge request !203