FAL-1838 tweaks iteration 1

FAL-1838

This makes some tweaks requested by the client (see feedback doc linked from ticket).

Test instructions:

• set up the frontend app at the revision from frontend-app-course-search!8 (merged)
• install this backend app in your local koa devstack and run ./manage.py lms course_search_init from the lms shell
• add a new course to studio
• verify that the new course is indexed, and that the publication date in the index is set to the publication date of the course (you can see this in the 'current es data' field in django admin for the course metadata, or the value returned from the search api)
• update the start date of the course in studio
• verify that the indexed publication date is updated
• set a publication date on the course metadata in django admin
• verify that the indexed publication date equals the publication date set in django admin
• view the course metadatas list in django admin (eg. http://localhost:18000/admin/backend_app_course_search/coursemetadata/ )
• verify that you can see the course id and name for each course, and that you can search to filter the courses there
• change the "Course Visibility In Catalog" on a course from 'both' to 'none' or 'about' (studio -> settings dropdown -> advanced settings)
• verify that the course no longer appears in search results

Reviewers:

• @tartansandal