Experiment: sqlx up/down migrations

[zaychenko-sergei]

Currenty we cannot revert a database migration in case anything goes wrong.
This causes a lot of deployment stabilization efforts when a mistake was made, including manual intervention into production db.

Experiment, if up/down migrations, offered by sqlx migrate commands, can help improving this aspect:

• sqlx migrate add -r: If true, creates a pair of up and down migration files with same version else creates a single sql file
• sqlx migrate revert: Revert the latest migration with a down file

It's assumed that any migration would require creating a reverse-migration to achieve the desired goal.
Open question: what do we do with the existing already applied migrations.