FIX back url when login to comment

[juanjosegzl]

Description

On the login url BackURL points to Parent.Link, which causes a bad redirect, e.g. if you are in /about-us the login link takes you back to /

I changed $Parent.Link to $Link

Manual testing steps

Go to a page other than homepage with comments, for example /about-us

The login link originally has BackURL=/

With the change it should show BackURL=/about-us

Pull request checklist

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• [] This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
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[GuySartorelli]

This module hasn't been in commercial support for a while, and I don't think it currently has any active maintainers.
If you'd like to maintain this module, please email community@silverstripe.org