Add new placeholders for group/project badges

Problem to solve

I want to be able to link a coverage badge to a report that is published with GitLab Pages. Unfortunately, there is no way to set this generically at the group level using the current placeholders.

%{project_path} includes the project's namespace, so there's no way to generate a URL like this:

http://<project_namespace>.pages.<my_gitlab_server>/<project_name>/coverage

Proposal

Add the following new placeholders:

• %{project_namespace} - the group/namespace of the project
• %{group_name} - the name of the group
• %{gitlab_server} (or similar) - e.g. example.com
• %{gitlab_pages_domain} (or similar) - e.g. pages.example.io

Links / references

https://docs.gitlab.com/ee/user/project/badges.html#placeholders

Implementation Guide

1. Update the app/models/badge.rb to include new placeholders (project_namespace, group_name, etc.) in the PLACEHOLDERS hash, e.g.:

'project_namespace' => ->(project) { project.project_namespace.to_param },
'group_name' => ->(project) { project.group&.to_param },
'gitlab_server' => -> { Gitlab.config.gitlab.dig('host') },
'gitlab_pages_domain' => -> { Gitlab.config.pages.dig('host') },

2. Update the app/assets/javascripts/badges/constants.js, to add support for the new placeholders.
3. Add relevant specs for the new changes
4. Update documentation for new placeholders (doc/api/group_badges.md, doc/api/project_badges.md and doc/user/project/badges.md).