Detect renamed repositories and broken links in landing pages

[achilleas-k]

This functionality can be a small, constantly running worker (goroutine) in gin-doi that periodically checks the status of dataset landing pages. Broken links can occur for mainly two reasons:

1. The original source repository was renamed.
2. The original source repository was deleted.

In the first case, the badge on the repository and the link in the fork on GIN will be intact. The link to the source repository on the landing page should be updated. This should be done by updating the XML file and regenerating the landing page through the make-html command. The published metadata should also be updated.

In the second case, the link should be removed. The procedure should be the same: Update the XML file (remove the relatedIdentifier), regenerate the landing page, and update the published metadata.

A related situation (which hasn't occurred so far) is the owner of the repository making it private. This would also make the DOI fork private. In this case, it might be better to treat it like the second case, but also edit the DOI fork to make it a non-fork (or recreate it).

[achilleas-k]

The service should probably also detect other types of broken links which would be a lot rarer but not triggered by user actions. This could detect errors with the data availability or page generation.