Move mailer preview to app or lib directory, and extract EE specific

We should try to move preview path from spec directory to somewhere else, because they're not used for specs. Also extract the EE specific part.

The following discussion from !5092 (merged) should be addressed:

• @MadLittleMods started a discussion:

  We can't put this in because a EE specific directory because we can only have one config.action_mailer.preview_path directory configured.

  Here is a PR to add functionality for multiple preview paths that I nudged, https://github.com/rails/rails/pull/31595