Require reserved domain password

[artur-intech]

• Make reserved_domains.password DB column NOT NULL
• Add tests
• Add hint in UI
• Remove dead code

Needed for #1062.

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Verify reserved and blocked domains logic.

[artur-intech]

Prod:

`registry=# select * from reserved_domains where password is null;
 id | created_at | updated_at | creator_str | updator_str | legacy_id | name | password 
----+------------+------------+-------------+-------------+-----------+------+----------
(0 rows)

registry=# select * from reserved_domains where password = '';
 id | created_at | updated_at | creator_str | updator_str | legacy_id | name | password 
----+------------+------------+-------------+-------------+-----------+------+----------
(0 rows)