[Data rearchitecture] Stop creating complete universe of article course timeslices

[gabina]

What this PR does

This PR stops calling create_timeslices_for_new_article_course_records when new articles are ingested. Having the complete universe of article course timeslices for courses with a huge number of articles implies a lot of disk space that is not useful since most timeslices are empty. For this reason, this PR implements a new approach, in which article course timeslices are only created if they are non-empty (i.e. when there is a revision for that article and that date). This new approach should considerably decrease the disk space used in the data-rearchitecture instance.

Open questions and concerns

This approach could be replicated for course user wiki timeslices in the future, although the course_user_wiki_timeslices table does not appear to take up considerable space.

[gabina]

Failing specs are not related to changes.

Failures:

  1) Assigned Articles view lets users submit feedback about articles
     Failure/Error: find('textarea.feedback-form').fill_in with: 'This is a great article!'

     Capybara::ElementNotFound:
       Unable to find css "textarea.feedback-form"
     # ./spec/features/assigned_articles_spec.rb:26:in `block (3 levels) in <top (required)>'
     # ./spec/features/assigned_articles_spec.rb:22:in `block (2 levels) in <top (required)>'