change 6-1-stable to 6-2-stable in installation guide

corrected the url of the init-script

Use init.d/gitlab script from 6-0-stable branch

Seems like init.d/gitlab script from master branch is not working anymore. Change to a version from 6-0-stable works fine.

It says:
```
Starting the GitLab Unicorn web server...
/etc/init.d/gitlab: 132: /etc/init.d/gitlab: script/web: not found
Starting the GitLab Sidekiq event dispatcher...
/etc/init.d/gitlab: 141: /etc/init.d/gitlab: script/background_jobs: not found
GitLab is not running.
```

Show milestones iid instead of id

Refactor search_autocomplete_source

Add logrotate config to update docs