Unread comments behavior

Catch-all issue for the following:
- https://github.com/Metaculus/metaculus/issues/4847
- https://github.com/Metaculus/metaculus/issues/4464
- https://github.com/Metaculus/metaculus/issues/3529
- https://github.com/Metaculus/metaculus/issues/4506
- https://github.com/Metaculus/metaculus/issues/4340

Some may already be fixed, needs investigation