Allow static asset serving from env variable (enhanced!)
Conflicts:
	railties/CHANGELOG.md

Closes #17945

`db:test:prepare` still purges the database to always keep the test
database in a consistent state.

This patch introduces new problems with `db:schema:load`. Prior
to the introduction of foreign-keys, we could run this file against
a non-empty database. Since every `create_table` containted the
`force: true` option, this would recreate tables when loading the schema.

However with foreign-keys in place, `force: true` wont work anymore and
the task will crash.

/cc @schneems

Fixes https://github.com/rails/rails/issues/17340.

Use Minitest::BacktraceFilter instead of removing all silencers. This
will allow the backtrace for all libraries in the plugin to be shown
while removing noise generated by Minitest's backtrace.

See https://github.com/rails/rails/commit/9b15828b5c347395b42066a588c88e5eb4e72279#commitcomment-8764492

If you run a generator such as:

```
rails generate model accounts supplier:references
```

The resulting migration will now add the corresponding foreign key
constraint unless the reference was specified to be polymorphic.

This reverts deprecations added in #13528.
The task is brought back for two reasons:
  1. Give plugins a way to hook into the test database initialization process
  2. Give the user a way to force a test database synchronization

While `test:prepare` is still a dependency of every test task, `db:test:prepare`
no longer hooks into it. This means that `test:prepare` runs before the schema
is synchronized. Plugins, which insert data can now hook into `db:test:prepare`.

The automatic schema maintenance can't detect when a migration is rolled-back,
modified and reapplied. In this case the user has to fall back to `db:test:prepare`
to force the synchronization to happen.

The tests to verify our testing setup used `RAILS_ENV=development`
to execute the tests. Let's keep it as close to a real-world setup
as possible.

Closes #17756

[Godfrey Chan, Zachary Scott]

This removes the unwanted deprecation warnings in the tests

This is a follow-up to 116de073. That change was for two reasons: 1) speed up the
tests by reducing the amount of log lines 2) avoid a deprecation warning from
a6de6f50.

This setting need to be on the basic app too, otherwise the deprecation warning
will show up on other unrelated test cases.

Creates an ApplicationMailer and layout by default, including html and body tags to reduce spam score

Just prefer secrets over config

- `secrets.secret_token` is now used in all places `config.secret_token` was
  - `secrets.secret_token`, when not present in `config/secrets.yml`,
    now falls back to the value of `config.secret_token`
  - when `secrets.secret_token` is set, it over-writes
    `config.secret_token` so they are the same (for backwards-compatibility)
  - Update docs to reference app.secrets in all places
    - Remove references to `config.secret_token`, `config.secret_key_base`
- Warn that missing secret_key_base is deprecated
- Add tests for secret_token, key_generator, and message_verifier
  - the legacy key generator is used with the message verifier when
    secrets.secret_key_base is blank and secret_token is set
  - app.key_generator raises when neither secrets.secret_key_base nor
    secret_token are set
  - app.env_config    raises when neither secrets.secret_key_base nor
    secret_token are set
- Add changelog

Run focused tests via
ruby -w -Itest test/application/configuration_test.rb -n '/secret_|key_/'

This reverts commit 482fdad5.

Fixes #17237.

https://travis-ci.org/rails/rails/jobs/39190983#L460

 1) Failure:
 AppGeneratorTest#test_psych_gem
 [test/generators/app_generator_test.rb:495]:
 Expected /gem 'psych',\s+'~> 2.0', \s+platforms: :rbx/ to match <snip>
 http://git.io/uuLVag)\ngem 'psych', '~> 2.0', platforms: :rbx\n\n

For the most part of cases --skip-gems option just remove one line in
the Gemfile. This is not worth for the complexity it adds.

Closes #17196

Closes #17197.
Closes #17207.

`{required}` is a type modifier so it should be:

    user:references{required}:index

and not:

    user:references:index{required}

It was changed to not set the cookie in get requests at
https://github.com/rails/turbolinks/commit/62cc3db457ad9745ccda047dd43ab84fb3d89707.

Related with dcb05f26.

 * The assertions in AS::Duration don't actually assert.
 * The assertion in Railtie will pass even when `eager_load_namespaces`
   doesn't include `AppTemplate::Application` if `Rails.application` is
   truthy.

For more details, see here:

 * https://github.com/rails/rails/pull/16998
 * https://github.com/rails/rails/pull/17000