Unify handling of default organization logos between CMS and LMS

Description

Currently, the logic for handling organization logos is split between the CMS and LMS. This separation causes issues with how the default organization logo URL is stored and resolved, especially when courses without logos are indexed and later displayed in the LMS. To address this inconsistency, the following changes are proposed.

Current Logic

CMS-specific:

• The value DEFAULT_ORG_LOGO_URL is used during course indexing.
• Course indexing is executed in the CMS.
• Course indexing saves the org logo url to the search engine index.

LMS-specific:

• The static url (part of the org logo url) must resolve inside LMS.
• The org logo url is part of the response returned by the course_discovery API endpoint in edx-search.
• The course discovery api is executed in the LMS.

Proposed Solution:

• Move DEFAULT_ORG_LOGO_URL to the LMS common settings.
• During course indexing, save None if an organization does not have a logo.
• In course discovery, before returning search results, check org_image_url: if None, use DEFAULT_ORG_LOGO_URL, which will now correctly resolve inside LMS.

Start by tracing course indexing in the CMS and the course discovery API in the LMS, focusing on DEFAULT_ORG_LOGO_URL and org_image_url. Verify how logo values are stored during indexing and resolved before search results are returned. Done means organizations without logos are indexed with None and course discovery substitutes the LMS-resolvable default URL.