Add configuration to enable mail previews

[lengarvey]

Adds config.action_mailer.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.

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

Adding this prevents devs from having to do stuff like:

  config.action_mailer.preview_path ||= defined?(Rails.root) ? "#{Rails.root}/test/mailers/previews" : nil

  routes.append do
    get '/rails/mailers'         => "rails/mailers#index"
    get '/rails/mailers/*path'   => "rails/mailers#preview"
  end

in order to enable mail previewing on staging.

[chancancode]

s/staging/development/?

[chancancode]

@pixeltrix does the overall feature seems fine to you?

[lengarvey]

I reworked this so there aren't any more changes inside railties/lib except to remove the route appending from the finisher. This also lets me place the configuration item in config.action_mailer instead of adding a global configuration item.

[rafaelfranca]

We need to update the configuration guides to add this new config.

[lengarvey]

I've updated the configuration guides to include both this new config and the existing config.action_mailer.preview_path config which didn't seem to be covered.

[guilleiguaran]

Please squash your commits on a single one