Stop using librest
librest is unmaintained and will not be ported to Soup 3.
We use librest in cases:
- When uploading changes to OSM POIs (to carry the OAuth token).
- Checking in using Foursquare.
We could probably implement the needed stuff from librest directly using libsoup (in JS). Also, possibly the Foursquare support (and check-in) might be removed, as it's not that reliable (seems to be quota issues).