Enable sorting within a group, when called by webpage user

Hello, a new sorting feature was recently implemented (dec 2019). It's great, but that works only for the initial call of BibBase.

However, when the user selects a feature from the "Group by", there does not seem to be a possibility to do any further sorting. 

It would be nice to be able to add options to the ways groups are displayed, maybe using the same sort feature already implemented. 

In particular, when the user clicks on "group by author" or "group by keyword", the order within each category is neither alphabetical (title or author), nor chronological, which is confusing to the end user (I understand that the order is dictated by the BibTex - we use the one from Zotero, so dont control the order of entries).

Thanks for the good work again!