refactor: Extract get_hot_topics.
refactor: Extract get_hot_topics.

This extraction will make a bit more sense when
we start doing bulk operations on a realm to
get digests, but even now, it encapsulates the
slightly complex way we cherry-pick the top 4
topics for a user.

hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,showell/zulip,punchagan/zulip,showell/zulip,andersk/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,showell/zulip,punchagan/zulip,showell/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,showell/zulip,showell/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,showell/zulip,kou/zulip,zulip/zulip