feat: groups placeholder

[pReya]

• new version of my last pr feat: add groups dynamic placeholder #579 (sorry, messed up the rebasing)
• allow {groups} as a dynamic placeholder for external sites (this is a very common scenario to do something/change content on the external site based on group membership)
• groups array elements will be joined with a comma and then url-encoded

[provokateurin]

Thanks @pReya!

If anyone else needs this and wants to make the separator configurable please open a PR.

[github-actions]

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