Add Composer Project Service (updated)

Created by: ikappas

This commit adds the per-project option to expose a project as a composer package. It generates and maintains a packages.json file along with a project specific /p/project-{id}.json file (conforming to https://getcomposer.org/doc/05-repositories.md#includes) in the public/ folder, allowing the GitLab install to be used as a Composer repository.

Note: There is a known issue integrating with the GitLab-Omnibus, namely that the public folder is owned by 'root' and GitLab is run as 'git'. The GitLab-Omnibus chef script needs to be updated to allow access to both packages.json and the /p/ folder in the public/ folder

It extends on the original idea providing extended service configuration options that allow to work with packages that do not include a composer.json

It has three operational modes allowing for:

1. Default: Searches for a composer.json file in the source code root folder and if found exports the package.
2. Project: Parse project attributes to create a virtual composer.json to use for exporting the package.
3. Advanced: Parse a custom json string supplied in the service configuration to export the package.

It also has branch / tag filters that allows fine-grained filtering of specific branches/tags to be exposed as composer packages.

In the process of providing a more complex configuration screen i added support for rendering fieldsets, and options in project service configuration along with enabling hints. Also notifications are enabled for Tag/Branch deletion from the gitlab host.

This is and updated merge request for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/106 working with the latest source code.

Please see feature thread http://feedback.gitlab.com/forums/176466-general/suggestions/5953067-provide-composer-package-repository-support

When a project that has activated the composer service receives a push, the public /packages.json file and project specific json file will be refreshed to include version definitions for the branches and tags as specified in the service configuration in the project's repository.