WIP: Add concurrent load test for feed generating query

[Aivean]

Add simple test that simulates concurrent feed generation requests like:

SELECT site_id FROM user_sites WHERE user_id = ?;

SELECT * from posts where site_id in (?) order by commented_at desc limit ?, ?;

Usage example (100 concurrent users, offset 1-8 pages, run for 20 seconds):

npm run load-test -- -u 100 -p 8 -t 20

Additional changes: moved all utility scripts into backend/utils directory.

[mugabe]

Path to migrations.js should be changed for production also.
And testing that the production docker container (or docker-compose.local.yml configuration) works correctly required, as utils dir might not be included now.