[Data rearchitecture] fix minor bug in UpdateTimeslicesCourseWiki

[gabina]

What this PR does

This PR fixes a minor bug found in data-rearchitecture-for-dashboard instance. The error case was found when the course start date was changed to a previous date, without the course having any edit. Therefore, ingestion start date remained the same as course start, and the timeslice for that date had not yet been created when UpdateTimeslicesCourseWiki ran.

Open questions and concerns

< anything you learned that you want to share, or questions you're wondering about related to this PR >

[gabina]

hmm HighQualityArticleMonitor specs are failing again.

Failures:

  1) HighQualityArticleMonitor.create_alerts_for_course_articles creates Alert records for edited Good articles
     Failure/Error: expect(HighQualityArticleAssignmentAlert.count).to eq(1)

       expected: 1
            got: 0

       (compared using ==)
     # ./spec/lib/alerts/high_quality_article_monitor_spec.rb:58:in `block (3 levels) in <top (required)>'

  2) HighQualityArticleMonitor.create_alerts_for_course_articles does create second Alert if the first alert is resolved and there are later assignments
     Failure/Error: expect(HighQualityArticleAssignmentAlert.count).to eq(2)

       expected: 2
            got: 1

       (compared using ==)
     # ./spec/lib/alerts/high_quality_article_monitor_spec.rb:128:in `block (3 levels) in <top (required)>'