Turbolinks 5 compatibility changes

Fix `assert_nothing_raised` deprecation warning format

[ci skip] Improve the readability of documents of nested_attributes

Fix typo in Form Helpers Guide

Fix `request.ssl?` bug with Action Cable

Before:

```
DEPRECATION WARNING: Passing arguments to assert_nothing_raised
        is deprecated and will be removed in Rails 5.1.
```

After:

```
DEPRECATION WARNING: Passing arguments to assert_nothing_raised is deprecated and will be removed in Rails 5.1.
```

This bug affects `wss://` requests when running Action Cable in-app.
Fixes #23620.

In order to fix issue #17621 we added a check to validations that
determined if a record should be validated. Based on the existing tests
and behavior we wanted we determined the best way to do that was by
checking if `!record.peristed? || record.changed? || record.marked_for_destruction?`

This change didn't make it into a release until now. When #23790 was
opened we realized that `valid?` and `invalid?` were broken and did not
work on persisted records because of the `!record.persisted?`.

While there is still a bug that #17621 brought up, this change was too
drastic and should not be a RC blocker. I will work on fixing this so
that we don't break `valid?` but also aren't validating parent records
through child records if that parent record is validate false. This
change removes the code changes to validate and the corresponding tests.
It adds tests for two of the bugs found since Rails 5 beta2 release.

Fixes #17621

Alternative to #23638 log to STDOUT via env var

otherwise we don't get the log messages we're looking for and the tests
fail intermittently.

Fix issue #23625

Merge pull request #23789 from wisetara/wisetara/deprecate-args-ActiveSupport__TestCase#assert_nothing_raised-for-pr

Wisetara/deprecate args active support  test case#assert nothing raised for pr

creating all databases mutates the connection pool.  This patch restores
the connection pool to the connection spec established before creating
all databases.  Fixes #23279

Improve Action Cable reconnection reliability

Ensures we don't get "onclose" events from a previous WebSocket that was in the "closing" state

We are seeing cases where the websockets get stuck in the 'closing' state
after a tab has been in background for a while. So lets treat those websockets
as closed.

We are creating the table but not deleting after the test.

Remove needless `drop_table :test_limits`

A `:test_limits` table has not been created.

Prevent ActionMailer initializer from triggering load of ActionMailer

Merge pull request #23419 from prathamesh-sonpatki/fix-showing-of-deprecation-warning-for-legacy-migrations

Correctly show deprecation warning for incompatible migrations

the after_initialize block has been changed to use the configuration from
`config.action_mailer` rather than `ActionMailer::Base` so that action mailer
is not loaded before it is necessary.
the mailer preview routes setup have been moved out of an `on_load(:action_mailer)`
block.

Action Cable readme fix [ci skip]