Add chromeless class to the wrapper when in chromeless mode.

This makes it easier for CSS rules to target chromeless mode. This is useful for xblocks which want to use some CSS tweaks specific to the chromeless view.

*JIRA ticket*: OSPR, TBD

*Dependencies*: None

*Sandbox URL*:

• LMS: https://pr15474.sandbox.opencraft.hosting/

• Studio: https://studio-pr15474.sandbox.opencraft.hosting/

*Partner information*: 3rd party-hosted open edX instance

*Merge deadline*: None

*Testing instructions*:

1. Use the Course Blocks API (`/api/courses/v1/blocks/?all_blocks=true&depth=all&course_id=Org/Course/Run`) to find a direct `student_view_url` link to a block (it will start with `/xblock/i4x://...`).
1. Visit the link and verify that `chromeless` class is present on the `courseware-wrapper` div.

Example from the sandbox: https://pr15474.sandbox.opencraft.hosting/xblock/block-v1:edX+DemoX+Demo_Course+type@problem+block@45d46192272c4f6db6b63586520bbdf4

*Reviewers*

• [x] @bdero

• [ ] edX reviewer[s] TBD