Add check for backlink presence in footnotes/endnotes

[martinpub]

Currently, we don't have a rule that checks backlink presence for footnotes/endnotes, so this should be added.

The current text of the Nordic Guidelines is not exact on which of the two methods for backlinks from the DAISY KB should be preferred in 2020-1, so to begin with, we should check both or any method (always including @role="doc-backlink"). Is link integrity check enough, or should we implement a cross-reference check that ensures that the backlink target is equal to the referencing noteref of the present footnote?

[martinpub]

Implement a rule including presence of backlink check, and also check target to be the correct footnote ref.

[josteinaj]

Was this solved by #509?

[martinpub]

Yes @josteinaj, this is solved by #509, I will close this now. Thanks!