Split up Catalog MFE DEPR and link to new `MFE_CONFIG` fallback DEPR in code

[brian-smith-tcril]

Overall DEPR for Legacy Home/Course Catalog/Course About pages: #36785

PR adding MFE_CONFIG fallbacks: #37130

MFE_CONFIG values that will be eventually DEPRECATED

ENABLE_COURSE_SORTING_BY_START_DATE

Current behavior:

1. pull ENABLE_COURSE_SORTING_BY_START_DATE from SiteConfiguration
2. Use backoff value from settings.FEATURES["ENABLE_COURSE_SORTING_BY_START_DATE"]

HOMEPAGE_PROMO_VIDEO_YOUTUBE_ID

1. pull homepage_promo_video_youtube_id from SiteConfiguration
2. Do not use any backoff value settings. Returns None as a backoff value.

HOMEPAGE_COURSE_MAX

1. pull HOMEPAGE_COURSE_MAX from SiteConfiguration
2. Use backoff value from settings.HOMEPAGE_COURSE_MAX

COURSE_ABOUT_TWITTER_ACCOUNT

1. pull course_about_twitter_account from SiteConfiguration
2. Use backoff value from settings.PLATFORM_TWITTER_ACCOUNT

NON_BROWSABLE_COURSES

1. Do not pull any values from SiteConfiguraion
2. Use the reversed value of settings.FEATURES.get("COURSES_ARE_BROWSABLE")

not settings.FEATURES.get("COURSES_ARE_BROWSABLE")

ENABLE_COURSE_DISCOVERY

1. Do not pull any values from SiteConfiguraion
2. Use the value from settings.FEATURES["ENABLE_COURSE_DISCOVERY"]