Remove users and groups command

Fixes #344

See merge request !257

Additional nginx server blocks

Enables optional addition of external vhosts to nginx configuration for multi-site hosts.

As can be seen by the many questions/blogposts on the topic of running multiple sites from a host configured by Gitlab Omnibus, this could be a welcome addition.

I've tried other ways (including separate nginx w/ passenger), but this small patch seems the most unobtrusive and easiest to implement.

BTW, there is probably a better include path to think of.

See merge request !254

Makefile comments

This change adds some comments to the Makefile to explain why we do certain things.

See merge request !256

Postinst external url

Fixes #354

See merge request !255

Gitlab shell configuration options

Fixes #291

`http_settings` has a default value of `{}` in gitlab-shell and `self_signed_cert` is [only used in if statement](https://gitlab.com/gitlab-org/gitlab-shell/blob/master/lib/gitlab_net.rb#L57).

`audit_usernames` and `log_level` all have default values in gitlab-shell.

See merge request !252

Use separate template for smtp_settings.rb  for Gitlab CI

As I mentioned in Issue #345, there is a bug in the generation of Gitlab CI smtp_settings.rb. It uses the same template as for gitlab. This commit introduces a separate template for CI.

See merge request !250

Update rugged and libgit2 to 0.21.2

This is needed because we have to match the version of rugged in gitlab-rails's Gemfile.

See merge request !253

link typo fix

add rebuild database indexes & cleanup

add details on backing up your SSH host keys

Fixed issue with Markdown link

* Add rebuild database indexes step to MySQL to PostgreSQL installations
* Use relative links where possible

Point at mysql_to_postgresql.md

Time zone

See merge request !251

Better to point at the other more up to date documate than replicate it here.

Users need to backup SSH keys if they want to do a complete restore to the same domain and not have users get the `WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!` and man-in-the-middle attack warning messages. Related to https://github.com/gitlabhq/gitlabhq/pull/8359.

changelog typo fixes

Add section for "Additional Configuration"

I had to dig in the source in order to figure out how to change the email_from on the CI server. I thought that this would be useful documentation for others afraid of source diving.

See merge request !248

Gitlab shell secret

See merge request !249