Add badge placeholder for pages unique domain

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Proposal

Currently there is no way for badges to link to pages if the project uses a unique domain. Since the unique domain is now set as default, there is no way for a badge to link to pages, unless you input the domain manually. This also makes it impossible for group badges to link to project pages.

I propose to solve this by adding a placeholder, e.g. `%{project_pages_unique_domain}, which renders to the project's unique domain.