[9.5] Disallow the `name` attribute on all user-provided markup

See merge request gitlab/gitlabhq!2166

- Add a how it works section where Herokuish is mentioned
- Make the notes more apparent
- Merge the postgres variables section with its parent

As a cache zip file with "/" upload seems to fail with 500 Internal Server Error.

commit 79858a631af97c2857700fa44c0e7ba09d621e12
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Fri Aug 25 23:33:04 2017 +0900

    Fix doc

commit d0c864c7f1522bd254458a7d6e1002352f34a143
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Tue Aug 22 22:27:32 2017 +0900

    Add tests

commit 1628eb8598f8ad0e6b573f3fb4fdcf5c3776885b
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Tue Aug 22 10:14:44 2017 +0000

    Update .gitlab-ci.yml

commit 440a3cdd0bb5d7b83f861f6d3762510b40df984a
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Fri Aug 18 14:46:56 2017 +0000

    Update build.rb

commit e925951e35c889bfc0520ce056e7df7befdeb93b
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Thu Aug 17 18:13:17 2017 +0900

    Add changelog

commit 7de5b4f50fc1ff6987c9a861c057c816d4fb285d
Author: Shinya Maeda <shinya@gitlab.com>
Date:   Thu Aug 17 18:11:53 2017 +0900

    ini

* Predefined variable represents the username of the GitLab user that started a build

* Predefined variable represents the name of the GitLab user that started a build

It was missing and expected it wouldn't hurt anyone

This API was mainly for internal usage, and has been moved to the
general API: APIv4. The endpoints have been deprecated since 9.0, and
won't see 10.0. :)