More robust PostgreSQL database duplication check

Evaluate `@active_storage_attached_#{name}` only once

Add `null: false` to Active Storage tables

Fixes #29045.

These columns aren't intended nullable.

Currently `"@active_storage_attached_#{name}"` in `define_method` is
evaluated every call. It is better to evaluate it only once.

Fix generator example code [ci skip]

Fix RDoc formatting: `+` doesn't work with `@` [ci skip]

refs: https://github.com/rails/rails/pull/30161

```
$ echo "+@size+" | rdoc --pipe
<p>+@size+</p>

$ echo "<tt>@size</tt>" | rdoc --pipe
<p><code>@size</code></p>
```

[ci skip]

[ci skip] Update testing.md [ci skip] 

Fix formatting of Active Storage docs [ci skip]

Follow up of #30188.

Fix formatting of Active Storage docs [ci skip]

* Use `+` instead of backquote.
* Remove escape from class to be link

Start `@reaper.run` after connection pool initialized

* Isolate ActiveStorage namespaces

* Rename migrations task

[Rafael Mendonça França + Dino Maric]

Otherwise `ConnectionPool#reap` may run before `@connections` has
initialized.

https://travis-ci.org/rails/rails/jobs/263037427#L888-L890

Improve railties generators tests

Fix random CI failure due to non-deterministic sorting order

Improve app generator tests.

Ensure that generation `config/application.rb` is correct.

Ensure that generation `config/application.rb` is correct.

Fix a typo in code comment [ci skip]

This reverts commit a8f59044.

See discussion on https://github.com/rails/rails/pull/30016.

This reverts commit fd7f978a.

Fix random CI failure DirtyTest

```
rails/activerecord$ bundle exec rake postgresql:test TESTOPTS="--seed=26536"

..(compressed)....F

Failure:
DirtyTest#test_save_should_not_save_serialized_attribute_with_partial_writes_if_not_present [/home/travis/build/rails/rails/activerecord/test/cases/dirty_test.rb:473]:
Expected nil to not be nil.
```

Fix broken RDoc formatting

The `@` ivar format doesn’t work with RDoc's `+` code formatting; needs `<tt>`.

[ci skip]

```
rails/activerecord$ bundle exec rake postgresql:test --verbose TESTOPTS="--seed=36062"

Failure:
AssociationsJoinModelTest#test_has_many_through_polymorphic_has_one
[/home/travis/build/rails/rails/activerecord/test/cases/associations/join_model_test.rb:407]:
--- expected
+++ actual
@@ -1 +1 @@
-[
   #<Tagging id: 1, tag_id: 1, super_tag_id: 2, taggable_type: "Post", taggable_id: 1, comment: nil>,
   #<Tagging id: 2, tag_id: 1, super_tag_id: nil, taggable_type: "Post", taggable_id: 2, comment: nil>
 ]
+#<ActiveRecord::Associations::CollectionProxy [
   #<Tagging id: 2, tag_id: 1, super_tag_id: nil, taggable_type: "Post", taggable_id: 2, comment: nil>,
   #<Tagging id: 1, tag_id: 1, super_tag_id: 2, taggable_type: "Post", taggable_id: 1, comment: nil>
 ]>
```

Fix engine command

Use the same key as presented in doc