feat: remove ENABLE_MKTG_SITE from enrollment, courseware, and mobile API

- course_about_url in enrollment emails now uses
  get_link_for_about_page(), resolving the marketing URL, social sharing
  URL, or LMS about URL as a fallback
- get_link_for_about_page() uses course.marketing_url unconditionally
- has_marketing_url() returns bool(self.marketing_url) directly
- url_to_enroll() always returns marketing_link('COURSES')
- _course_home_redirect_enabled() only checks ENABLE_COURSE_HOME_REDIRECT;
  default changed from True to False because the flag previously had no
  effect when ENABLE_MKTG_SITE=False (the old default) — operators who
  relied on ENABLE_MKTG_SITE=True to activate the course-about redirect
  must now set ENABLE_COURSE_HOME_REDIRECT=True explicitly
- Remove ENABLE_MKTG_SITE patches from courseware, instructor, and
  mobile API tests; merge redundant test cases where applicable

BREAKING CHANGE: ENABLE_COURSE_HOME_REDIRECT now defaults to False.
Operators who previously had ENABLE_MKTG_SITE=True and want the
course about page to redirect to the learning MFE must explicitly set
ENABLE_COURSE_HOME_REDIRECT=True in their configuration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>