* WIP: Add credentials using a generic EncryptedConfiguration class

This is sketch code so far.

* Flesh out EncryptedConfiguration and test it

* Better name

* Add command and generator for credentials

* Use the Pathnames

* Extract EncryptedFile from EncryptedConfiguration and add serializers

* Test EncryptedFile

* Extract serializer validation

* Stress the point about losing comments

* Allow encrypted configuration to be read without parsing for display

* Use credentials by default and base them on the master key

* Derive secret_key_base in test/dev, source it from credentials in other envs

And document the usage.

* Document the new credentials setup

* Stop generating the secrets.yml file now that we have credentials

* Document what we should have instead

Still need to make it happen, tho.

* [ci skip] Keep wording to `key base`; prefer defaults.

Usually we say we change defaults, not "spec" out a release.

Can't use backticks in our sdoc generated documentation either.

* Abstract away OpenSSL; prefer MessageEncryptor.

* Spare needless new when raising.

* Encrypted file test shouldn't depend on subclass.

* [ci skip] Some woordings.

* Ditch serializer future coding.

* I said flip it. Flip it good.

* [ci skip] Move require_master_key to the real production.rb.

* Add require_master_key to abort the boot process.

In case the master key is required in a certain environment
we should inspect that the key is there and abort if it isn't.

* Print missing key message and exit immediately.

Spares us a lengthy backtrace and prevents further execution.

I've verified the behavior in a test app, but couldn't figure the
test out as loading the app just exits immediately with:

```
/Users/kasperhansen/Documents/code/rails/activesupport/lib/active_support/testing/isolation.rb:23:in `load': marshal data too short (ArgumentError)
	from /Users/kasperhansen/Documents/code/rails/activesupport/lib/active_support/testing/isolation.rb:23:in `run'
	from /Users/kasperhansen/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/minitest-5.10.2/lib/minitest.rb:830:in `run_one_method'
	from /Users/kasperhansen/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/minitest-5.10.2/lib/minitest/parallel.rb:32:in `block (2 levels) in start'
```

It's likely we need to capture and prevent the exit somehow.
Kernel.stub(:exit) didn't work. Leaving it for tomorrow.

* Fix require_master_key config test.

Loading the app would trigger the `exit 1` per require_master_key's
semantics, which then aborted the test.

Fork and wait for the child process to finish, then inspect the
exit status.

Also check we aborted because of a missing master key, so something
else didn't just abort the boot.

Much <3 to @tenderlove for the tip.

* Support reading/writing configs via methods.

* Skip needless deep symbolizing.

* Remove save; test config reader elsewhere.

* Move secret_key_base check to when we're reading it.

Otherwise we'll abort too soon since we don't assign the secret_key_base
to secrets anymore.

* Add missing string literal comments; require unneeded yaml require.

* ya ya ya, rubocop.

* Add master_key/credentials after bundle.

Then we can reuse the existing message on `rails new bc4`.

It'll look like:

```
Using web-console 3.5.1 from https://github.com/rails/web-console.git (at master@ce985eb)
Using rails 5.2.0.alpha from source at `/Users/kasperhansen/Documents/code/rails`
Using sass-rails 5.0.6
Bundle complete! 16 Gemfile dependencies, 72 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
Adding config/master.key to store the master encryption key: 97070158c44b4675b876373a6bc9d5a0

Save this in a password manager your team can access.

If you lose the key, no one, including you, can access anything encrypted with it.

      create  config/master.key
```

And that'll be executed even if `--skip-bundle` was passed.

* Ensure test app has secret_key_base.

* Assign secret_key_base to app or omit.

* Merge noise

* Split options for dynamic delegation into its own method and use deep symbols to make it work

* Update error to point to credentials instead

* Appease Rubocop

* Validate secret_key_base when reading it.

Instead of relying on the validation in key_generator move that into
secret_key_base itself.

* Fix generator and secrets test.

Manually add config.read_encrypted_secrets since it's not there by default
anymore.

Move mentions of config/secrets.yml to config/credentials.yml.enc.

* Remove files I have no idea how they got here.

* [ci skip] swap secrets for credentials.

* [ci skip] And now, changelogs are coming.