Against a clean seeded DB, reduces `GET /post/1/` from 63 queries to
26 by removing redundancies and slow lazy-loaded queries during
top comment pagination.
Also applies eager loading to /viewmore/ with the expected reduction
from 5*(N comments) queries to ~12/request.
For testing locally, use a newly seeded DB to ensure
Comment.descendant_count is populated.
Ref: #485
007f0a3f02