Add new placeholders for group/project badges

Description

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
• %{project_name} - the name of the project
• %{gitlab_server} (or similar) - e.g. example.com
• %{gitlab_pages_domain} (or similar) - e.g. pages.example.io

This would also be nice for unrelated reasons:

• %{current_branch}

Links / references

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