Allow edits of existing encrypted secrets generated on Rails 5.1,
but refer to credentials when attempting to setup.
This also removes the need for any of the setup code, so the
generator can be ripped out altogether.
In encrypted secrets, the tmp file is used as a fixed file
(`secrets.yml.enc` under the tmp directory).
And that tmp file will be removed after process.
Therefore, if edit secrets at the same time with multiple applications,
the tmp file was conflicting.
In order to avoid the above issue, added pid to tmp file.
Currently, if open a file with `secrets:edit` command, `secrets.yml.enc`
will be changed even if its contents do not change.
Therefore, even if only want to check secrets, the difference will come
out. This is a little inconvenient.
As a fix to the above problem, when content does not change,
`secrets.yml.ecn` is fixed so that it is not changed.
Currently, `read_encrypted_secrets` is set with initializer.
Therefore if refer to `secrets` in config, `read_encrypted_secrets` is false,
so can not get the value of `secrets.yml.enc`.
In order to be able to refer to secrets in config, modified to refer to
`config.read_encrypted_secrets` when calling `secrets`.
Fixes#28618.
