Remove "the" [ci skip]

Add test validating that Model.attribute_names cache is busted

Since we have `Preloader#preload`, `Preloader::Association#preload` is a
little confusing. And also, since the `preload` method is an abstract
method, it is hard to read where `associated_records_by_owner` is
called. This refactors `Preloader::Association` to ease to read where
`associated_records_by_owner` is called.

Remove unused require in ActiveStorage::Variation

If collided named sequence already exists, newly created serial column
will generate alternative sequence name. Fix sequence name detection to
allow the alternative names.

`options` is never assigned to `scope` as long as using splat hash.

Class level `update` and `destroy` are using `find` in the internal, so
it will raise `RecordNotFound` if given ids cannot find an object even
though the method already affect (update or destroy) to any objects.
These methods should return affected objects even in that case.

The docs are obviously for class level `update`, `destroy`, and
`delete`. It should be placed in `Persistence::ClassMethods` rather than
`Relation`. And also, these methods are not dependent on relation. So it
is not needed to delegate to `all` (plus, `klass.find` is faster than
`relation.find`).

Related 2b5f5cdd.

If `reflection.scope` isn't given, `reflection_scope` is always empty
scope. It is unnecessary to merge it.

Related 2b5f5cdd.

If `through_scope` is empty scope, it is unnecessary to merge it.
And also, comparing relations is a little expensive (will cause
`build_arel`). It is enough to use `empty_scope?` to determine whether
empty scope.

`through_scope` is not empty scope if `options[:source_type]` is given.

Add missing round bracket in "Ruby on Rails 5.1 Release Notes" guide [ci skip]

[ci skip]

Add `mini_magick` to default `Gemfile` as comment

Fix docs describing rollback [ci skip]

* If we want to transform image on ActiveStorage, we should bundle
  `mini_magick`.
* I've added comment block to default `Gemfile` to be easier to install this.

* `rails db:migrate STEP = 2` will not rollback the migrations, instead
   `rails db:rollback STEP = 2` will do the rollback.
* Also, rewritten `rails db:migrate VERSION` => `rails db:rollback VERSION`
  for consistency.

Update plugins.md [ci skip]

Add local option to Message form [ci skip]

Fix file missing in activestorage's example code [ci skip]

* File.open("~/face.jpg") raise error:
  `Errno::ENOENT: No such file or directory @ rb_sysopen - ~/face.jpg`

Add tests for credentials command

* MessagesController redirects to `GET /message/:id`.
* It looks it don't expect XHR request.
* `form_with` behaves for XHR by default.
* I've added `local: true` option to `form_with`.

Remove frozen_string_literal magic comment from templates

Follow up of #30348

Don't expose Active Storage routes

Fix typo: `credentails` -> `credentials` [ci skip]

Follow up of ca18922a

These routes are only used internally in Active Storage, and it seems
that there is no need for the user to directly use them.

Therefore, I think that routes should not be exposed to users.

Update Getting Started with Rails [ci skip]