Add #create_or_find_by to lean on unique constraints

Use require_dependency inside Active Storage

Add support for connection pooling on RedisCacheStore

Active Storage is an engine which means its models, jobs and controllers
are autoloaded by Rails rather than Ruby. Unfortunately this means it's
subject to the same gotchas as applications, including this one:

http://guides.rubyonrails.org/v5.1.4/autoloading_and_reloading_constants.html#when-constants-aren-t-missed-qualified-references

On Ruby < 2.5, constants nested under classes can't be autoloaded by
Rails if a top level constant already exists with the same name.

To avoid clashing with constants defined in users' applications or gems,
we can use `require_dependency` to ensure that the nested constants are
loaded before they're used.

Introduce custom serializers to ActiveJob arguments

This will make easier to be backwards compatible when changing the
serialization implementation.

We can speed up things for the supported types by keeping the code in the
way it was.

We can also avoid to loop trough all serializers in the deserialization by
trying to access the class already in the Hash.

We could also speed up the custom serialization if we define the class
that is going to be serialized when registering the serializers, but
that will remove the possibility of defining a serialzer for a
superclass and have the subclass serialized using it.

Now custom serialziers can register itself in the serialized hash using
the "_aj_serialized" key that constains the serializer name.

This way we can avoid poluting the hash with many reserved keys.

Right now it is only possible to define serializers globally so we don't
need to use a class attribute in the job class.

Rdoc formatting fix: <b> instead of MD-style asterisks

[ci skip]

Bump i18n to 1.0

Don't overwrite config/master.key even on --force

See https://github.com/rails/rails/pull/31957#issuecomment-364817423

The purpose of `--force` is not to have any prompt whether a file should
be kept or overwritten. In general, all existing files should be overwritten.
However, `config/master.key` is special because it is git-ignored, and
overwriting it will cause the app not to run (since there won't be a way
to decrypt the credentials).

As a result, it's probably better to keep the existing config/master.key.

Add ActiveStorage::Blob.unattached scope

Incompatible settings are included in the settings set by `load_defaults`.
So, I think that target version should be updated by a user when becomes
available, and should not be updated with `app:update`.

[ci skip]

Closes #31967

Change structure in the AS core extensions guide [ci skip]

ActiveStorage: use full class names when including concerns to avoid collisions

Improve wording in getting started guide

Missing parentheses [ci skip]