Rework changelog widget unviewed items

Currently, there are a couple of problems with the changelog widget:

TRACK VIEWS PER USER

We don’t track changelog views per user. It’s currently per device.

Ideally, everything should be tracked per user and fall back to the per-device setup if no user is provided.

MAKE UNVIEWED CHANGELOG COUNT BETTER

Currently, the unviewed changelog count implementation is bad and could be improved.

Posts talking about issues with the count:

• @fb-update-badge - event as well?

Getting the unviewed changelog count via the SDK could work for more advanced setups:

Featurebase(“getunviewedchangelog_count”, {…})

We should probably emit an event if the unviewed changelog count changes so people with more advanced setups can listen for the change and display the count accordingly.