ActiveStorage::Filename is a model, so its test case belongs in test/models.

If created active storage tables by mysql2 or postgresql adapters, a
primary key is defined as a bigint. It should be used `references` to
the reference columns to respect primary key type.

Active Storage was now merged to master and it is automatically
installed in a new application.

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.

Follow up of #30188.

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

* Isolate ActiveStorage namespaces

* Rename migrations task

[Rafael Mendonça França + Dino Maric]

Fixes that ActiveStorage::Blob.service is unset when ActiveStorage::Blob
is reloaded.

* Add migrations per rails engine conventions

* Fix failing tests

Add in the content_type option, which is defined as part of the generic
ActiveStorage::Service class.

Without this option, attempts to generate a URL for an attached file
fail with "ArgumentError (unknown keyword: content_type)"

* Extend image_tag to accept ActiveStorage's Attachments and Variants

* Flip resolve_image_source around

* Add tests for the new use-cases of image_tag

* Remove the higher-level test

* Update image_tag documentation

* Add error states into the test suite

* Re-raise polymorhic_url's NoMethodError as ArgumentError

* delegate_missing_to will raise DelegationError instead of NoMethodError

Filter out Minitest backtrace while allowing backtrace from other
libraries to be shown.

Follow #30083

By default, apps only have the former set.

This fixes following warnings:

```
test/models/variant_test.rb:11: warning: ambiguous first argument; put parentheses or a space even after `/' operator
lib/active_storage/attached/macros.rb:63: warning: instance variable @active_storage_attached_highlights not initialized
lib/active_storage/attached/macros.rb:25: warning: instance variable @active_storage_attached_avatar not initialized
```

The reasons for this commit are:

- uniformity with the other Rails libraries
- (possibly) behave better with respect to autoloading
- fix the index in the generated documentation

Before this commit, run `rake rdoc` generates this left sidebar
(ActiveStorage entries are indexed twice, both inside and outside the module):

<img width="308" alt="before" src="https://user-images.githubusercontent.com/10076/28939523-7c087dec-7846-11e7-9289-38ed4a2930cd.png">

After this commit, run `rake rdoc` generates this left sidebar:
(ActiveStorage entries are only indexed inside the module):

<img width="303" alt="after" src="https://user-images.githubusercontent.com/10076/28939524-7c090be0-7846-11e7-8ee5-29dfecae548e.png">

changed `rails/activestorage` to `rails/rails`.

`start_with?` is a little faster than regexp for prefix matching by a
fixed string.

If an AWS bucket name includes a `.` (e.g. `bucket.name`), then the canonical
URL for an object will start with "https://s3.amazonaws.com/bucket.name/"
and not with "https://bucket.name.s3.amazonaws.com/".

The URL tests have now been separated into two separate asserts, to ensure
that both the "s3.amazonaws.com" and the "bucket.name" components are included,
but not specifically in that order.

We are already removing the braces around hash parameters in the last
argument in other places so we should not change the entire codebase
because of two places.

Everything inside the app directory of a engine is autoload/eager loaded automatically so we don't need to require them.

I know those methods are unlikely to change but having one line method
is hard to read and also hard to modify.