Make purging unread items an optional feature

[Grotax]

IMPORTANT

Read and tick the following checkbox after you have created the issue or place an x inside the brackets ;)

• [x] I have read the CONTRIBUTING.md and followed the provided tips
• [x] I accept that the issue will be closed without comment if I do not check here
• [x] I accept that the issue will be closed without comment if I don not fill out all items in the issue template.

Description

In older versions of News read items where deleted if the amount of read items exceeded a certain threshold defined in the admin settings. Current version doesn't delete anything, News 15.2.0 will feature the old behavior in a refactored version.

The next minor version will introduce a new option to also delete unread items, to make it easier for administrators to keep their instance clean.

Task

• add option to delete unread && read items
• announce via changelog

[Grotax]

related #1065

[anoymouserver]

also related to #110

[Grotax]

done in #1931