Editing / CMS integration

[glebm]

Currently sofa support is hacked in here:

• /lib/rails_email_preview/integrations/comfortable_mexica_sofa.rb: mailer helpers, hooks, and configuration
• /app/views/integrations/cms/_customize_cms_for_rails_email_preview.html.slim: view integration

A better approach is a CMS-indepenent view and model layer, with modular storage, mailer, and view extensions. Would be nice to have a default storage engine option as well. PRs accepted.