Use a single document for upgrading from CE to EE

This changes the guides for upgrading from CE source to EE source so we
no longer need separate documents for every version. This removes the
need for release managers to create and update these documents, and
allows developers to add these instructions as part of their code
changes.

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+---
+comments: false
+---
+
+# Upgrading from Community Edition to Enterprise Edition from source
+
+NOTE: **NOTE** In the past we used separate documents for upgrading from
+Community Edition to Enterprise Edition. These documents can be found in the
+[`doc/update` directory of Enterprise Edition's source
+code][old-ee-upgrade-docs].
+
+## General upgrading steps
+
+This guide assumes you have a correctly configured and tested installation of
+GitLab Community Edition. If you run into any trouble or if you have any
+questions please contact us at [support@gitlab.com].
+
+In all examples, replace `EE_BRANCH` with the Enterprise Edition branch for the
+version you are using, and `CE_BRANCH` with the Community Edition branch.
+Branch names use the format `major-minor-stable-ee` for Enterprise Edition, and
+`major-minor-stable` for Community Edition. For example, for 11.8.0 you would
+use the following branches:
+
+* Enterprise Edition: `11-8-stable-ee`
+* Community Edition: `11-8-stable`
+
+### 0. Backup
+
+Make a backup just in case something goes wrong:
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
+```
+
+For installations using MySQL, this may require granting "LOCK TABLES"
+privileges to the GitLab user on the database version.
+
+### 1. Stop server
+
+```bash
+sudo service gitlab stop
+```
+
+### 2. Get the EE code
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H git remote add -f ee https://gitlab.com/gitlab-org/gitlab-ee.git
+sudo -u git -H git checkout EE_BRANCH
+```
+
+### 3. Install libs, migrations, etc.
+
+```bash
+cd /home/git/gitlab
+
+# MySQL installations (note: the line below states '--without postgres')
+sudo -u git -H bundle install --without postgres development test --deployment
+
+# PostgreSQL installations (note: the line below states '--without mysql')
+sudo -u git -H bundle install --without mysql development test --deployment
+
+# Run database migrations
+sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
+
+# Clean up assets and cache
+sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS_ENV=production
+```
+
+### 4. Install `gitlab-elasticsearch-indexer` (optional) **[STARTER ONLY]**
+
+If you're interested in using GitLab's new [elasticsearch repository
+indexer][indexer-beta] (currently in beta) please follow the instructions on the
+document linked above and enable the indexer usage in the GitLab admin settings.
+
+### 5. Start application
+
+```bash
+sudo service gitlab start
+sudo service nginx restart
+```
+
+### 6. Check application status
+
+Check if GitLab and its environment are configured correctly:
+
+```bash
+sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production
+```
+
+To make sure you didn't miss anything run a more thorough check with:
+
+```bash
+sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
+```
+
+If all items are green, then congratulations upgrade complete!
+
+## Things went south? Revert to previous version (Community Edition)
+
+### 1. Revert the code to the previous version
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H git checkout CE_BRANCH
+```
+
+### 2. Restore from the backup
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:restore RAILS_ENV=production
+```
+
+## Version specific steps
+
+Certain versions of GitLab may require you to perform additional steps when
+upgrading from Community Edition to Enterprise Edition. Should such steps be
+necessary, they will listed per version below.
+
+<!--
+Example:
+
+### 11.8.0
+
+Additional instructions here.
+-->
+
+[support@gitlab.com]: mailto:support@gitlab.com
+[old-ee-upgrade-docs]: https://gitlab.com/gitlab-org/gitlab-ee/tree/11-8-stable-ee/doc/update
+[indexer-beta]: https://docs.gitlab.com/ee/integration/elasticsearch.html