Allow PDF TN page be configurable in the derivative action

[rosiel]

Follow-up to the awesome, time-saving #2165 :

Overview of feature request
Sometimes repo PDFs such as ETDs contain cover pages that make bad thumbnails.

What kind of user is the feature intended for?
Collections managers, specifically IR managers

What inspired the request?
#2165

What existing behavior do you want changed?
Right now the behaviour is hard coded to get only the first PDF page. We'd like the page number to be configurable in the System Action.

This way if you want certain objects/collections to use one configuration (page 1) and another collection to use a different configuration (page 2) you could do that by defining two different derivative Actions, and use Contexts to select the appropriate one.

Any related open or closed issues to this feature request?
#2165