Make previews controller extensible

[juanmanuelramallo]

Feature request

Make the previews controller to be extensible by allowing users to add authentication, authorization, before actions, or anything that the end user would need to meet their needs.

Motivation

It all started in discussion #412
@jonspalmer came up with a good suggestion, which is to copy Devise's "parent class" feature. It will enable devs to set up a parent class if needed.

#412 (reply in thread)

[swanson]

👋 Going to take a stab at this.