Let's say you have a nested transaction and both records are saved.
Before the outer transaction closes, a rollback is performed. Previously
the record in the outer transaction would get marked as not persisted
but the inner transaction would get persisted.

```ruby
Post.transaction do
  post_one.save # will get rolled back

  Post.transaction(requires_new: true) do
    post_two.save # incorrectly remains marked as persisted
  end

  raise ActiveRecord::Rollback
end
```

To fix this the PR changes transaction handling to have the child
transaction ask the parent how the records should be marked. When
there are child transactions, it will always be a SavpointTransaction
because the stack isn't empty. From there we pass the parent_transaction
to the child SavepointTransaction where we add the children to the parent
so the parent can mark the inner transaction as rolledback and thus mark
the record as not persisted.

`update_attributes_from_transaction_state` uses the `completed?` check to
correctly mark all the transactions as rolledback and the inner record as
not persisted.

```ruby
Post.transaction do
  post_one.save # will get rolled back

  Post.transaction(requires_new: true) do
    post_two.save # with new behavior, correctly marked as not persisted
    on rollback
  end

  raise ActiveRecord::Rollback
end
```

Fixes #29320

`set_state` was directly setting the transaction state instance
variable. It's better to set the state via specific methods (`rollback!`
and `commit!` respectively.

While undocumented and untested, it's possible someone is using
`set_state` in their app or gem so I've added a deprecation notice to
it.

No where in the app do we use `nullify!` but I wanted to keep existing
behavior while replacing the method with a better pattern.

Warning http verb method call in SystemTestCase

This reverts commit 3420a145, reversing
changes made to afb66a5a.

Enforce frozen string in Rubocop

Make ActiveSupport frozen-string-literal friendly.

Add srcset option to image_tag helper

Clear screenshots files in `tmp:clear` task

Add source code and changelog link to railties.gemspec

Should be clear `@association_ids` when joined newly associated record

Don't cache queries for schema statements

Properly register "custom" URL helpers as named helpers.

Fix automatic inverse for polymorphic interfaces

Ensure `false` is preserved in attr serialization

Fix the next version of Rails from 5.3 to 6.0

Fix format of `ActionController::Parameters#to_s` doc [ci skip]

Delete stale comment for `AR::Associations::Builder::CollectionAssociation` [ci skip]

CustomUrlHelpers were introduced in ce7d5fb2, closing issue #22512.
They currently register themselves in an ivar that is never accessed.
This change removes the @custom_helpers special-case, and registers them
the way named routes are normally handled.

Without this, you can get route_defined?(:example_url) == false, while
still being able to call url_helpers.example_url and example_path.

Various popular gems such as 'rspec-rails' make use of route_defined?()
when determining how to proxy method calls or whether to define a route.

Fallback Parameters#to_s to Hash#to_s

Add system tests to generate scaffold

Fixes https://github.com/rails/rails/issues/29617

[ci skip] run migration before looking at schema

`test_middleware_caches` is sometimes failed since #29454.
The failure is due to schema statements are affected by query caching.
Bypassing query caching for schema statements to avoid the issue.

Fixes #29627.

Specify correct extension for text layouts

Should use the same connection in using query cache

Fix the next version of Rails from 5.3 to 6.0

Deprecate delegating to `arel` in `Relation`