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.
When secrets confirmed with the `secrets:edit` command, `secrets.yml.enc`
will change without updating the secrets.
Therefore, even if only want to check secrets, the difference will come
out. This is a little inconvenient.
In order to solve this problem, added the `secrets:show` command.
If just want to check secrets, no difference will occur use this command.
In case there's no $EDITOR assigned users would see a cryptic:
```
% EDITOR= bin/rails secrets:edit
Waiting for secrets file to be saved. Abort with Ctrl-C.
sh: /var/folders/wd/xnncwqp96rj0v1y2nms64mq80000gn/T/secrets.yml.enc: Permission denied
New secrets encrypted and saved.
```
That error is misleading, so give a hint in this easily detectable case.
Fixes#28143.
