DB schema generated by a migration may look different in
rails 4 and 5 (because rails 5 may use different default values).
For this reason it's important to explicitly set for which rails
version a migration was written for.

See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912

Signed-off-by: Rémy Coutable <remy@rymai.me>

This is to ensure that migrations will still be consitent when we will
upgrade to Rails 5 which default to `null: false` for timestamps
columns.

Fixes #23666.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Migrations shouldn't fail RuboCop checks - especially lint checks, such
as the nested method check. To avoid changing code in existing
migrations, add the magic comment to the top of each of them to skip
that file.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>