Add configuration to enable mail previews

Adds `config.mail_preview_enabled`

This allows mail previewing to be enabled easily in non-development
environments such as staging. The default is set to true for development
so no changes should be required to existing Rails applications.

I added this as a new config option in railties since it's required in
both actionmailer and in the railtie finisher. Otherwise we'd be placing
a dependency on `config.action_mailer.preview_path` in order to enable
mail previews.

The mail preview path can still be configured using the existing
`config.action_mailer.preview_path` configuration option.

Adding this avoids devs from having to do stuff like:
https://gist.github.com/lengarvey/fa2c9bd6cdbeba96526a