An MSID can now optionally be returned by the /submit-listen  endpoint when submitting a playing-now listen, allowing to send feedback.
This was added in https://github.com/metabrainz/listenbrainz-server/pull/3513

However this is not always available when looking at someone's profile.
We should add an API endpoint that accepts:

1. user_name
2. artist_name
3. track_name
4. score (1, -1)

This endpoint can then lookup the MSID and write a feedback entry, enabling us to like/hate tracks that we haven't listened to.

IRC discussion: https://chatlogs.metabrainz.org/libera/metabrainz/2021-09-17/?msg=4861062&page=1