Create frontend-base tutor-mfe plugin

[arbrandes]

Description

Create a plugin to tutor-mfe that:

1. Upon a rebuild of the mfe image, includes the frontend-template-site build artifacts. It does not remove any existing MFEs, just adds the frontend-base ones.
2. Exposes site.config in a similar way to how env.config.jsx is exposed today.
3. Modifies Django config so that the frontend-base MFEs are used by default when the plugin is enabled.

In addition, in dev mode it should be possible to easily disable the frontend-base site's port binding in a way similar to how bind-mounting an MFE and not starting its dev image does it. This is so it'll possible to develop frontend-base MFEs by running npm run dev outside the Tutor Docker container, but still having it work.