Feature: metric for grouped links by newsletter

[rr-it]

Enable custom metric "top 10 links by newsletter".

[rr-it]

"[Feature] metric for grouped links by newsletter" https://github.com/in2code-de/luxletter/pull/127 implements the base for "[POC] top ten links by newsletter" https://github.com/in2code-de/luxletter/pull/128

Right now for links there is only one ranking over all newsletters ever sent. E.g. links from 2020 are compared to links from 2022.

With this feature:

• The overall link ranking still exists.
• Additionally display the top 10 links for the last 10 newsletters.

[einpraegsam]

As talked before. On the one hand we are discussing from time to time to add an additional view for additional analyses, but this would be handled in a bit different way. So for now I would tend to close this issue.