There were two issues:
- first, `load_or_fetch_event_with_relations()` allowed to pass a
filter, but the filter wasn't taken into account when fetching relations
from the network. This would cause the initial load of pinned events to
also include thread responses, which we don't want.
- similarly, when adding related events from sync, we'd only look if an
event had a `m.relates_to` field; but it could be a thread response
being added in live.
The two issues are fixed similarly, by using a new `extract_relation`
serde helper that gives both the related_to event and the relation type.
That way, we can apply a manual filter in
`load_or_fetch_event_with_relations` after fetching relations from
network, and we can filter out live events based on the relation type.
Benjamin Bouvier
071f982eed