This commit enhances the `Rails::Command::Base.executable` method to
integrate a new `bin` class attribute and optional `subcommand` arg.
So, for example, `CredentialsCommand.executable(:edit)` will now return
"bin/rails credentials:edit".

Additionally, this commit replaces occurrences of "bin/rails COMMAND",
"rails COMMAND", and "COMMAND" in help messages with calls to
`executable`, for improved consistency.