🚨 Add CI linter on Cunningham theme

## Feature Request

After an upgrade of "@gouvfr-lasuite/ui-kit" or "@openfun/cunningham-react" the generated theme can have changes. If the developer forget to rebuild the theme, we can get missed or not updated properties.

Add a linter to check that the theme is correctly updated by checking that there is not git diff between the versioned theme and the CI generated one.