Next/Prev entry links

[johanfforsberg]

The Entry.next/prev links are supposed to point to chronologically neighboring entries (basically the same order as in the logbook entry list, but per logbook). They are currently kind of buggy in the database layer, and therefore not used in the UI. Fix them and use them to make it possible to use them to navigate.

It would also be nice to have some similar functionality to have links that scroll to the next/previous followup in the entry view, but that can probably be done in the frontend.