Add `behind_the_scenes_for_id` to EphemeraFolders

[tpendragon]

Summary or User Story

We're starting to generate images that document the behind the scenes of digitizing a resource and creating Ephemera Folders for those. We'd like to link those in Digital Collections, but to do that we need a stable ID.

Acceptance Criteria

• There's a new field behind_the_scenes_for_id in EphemeraFolders.
• That field is editable in Figgy. The label is "Behind the Scenes For", and the informational text is "If this folder is a behind the scenes photo for another resource, put the Figgy ID here."

First step

Write a test.