Add support for sub-themes of Gin

Problem/Motivation

I'm using a sub-theme of Gin to further customize the backend. This module works great when I set the theme to Gin, but when I use my custom sub-theme of Gin, the changes from this module no longer apply and Site Studio breaks again.

Steps to reproduce

1. Create a sub-theme of Gin, and set it to be the active Administration theme.
2. Enable site studio for any field on a content type
3. Attempt to edit a Site Studio component, it will be broken again

Proposed resolution

Instead of checking if the active theme is Gin, also check if Gin is listed as a base theme for the active theme.

Remaining tasks

Update logic to check if Gin is active theme, or a base theme for the active theme.

User interface changes

None

API changes

None

Data model changes

None