fix: exclude anonymous posts in build_course_stats

[ayesha-waris]

INF-887

Explanation:

When a user makes their first non-anonymous post, the system uses the "build_course_stats" function to create a new set of statistics for that user. However, there is a problem with the current implementation of the function. It counts all the posts made by a specific user, including those that are anonymous or were made anonymously to peers, before the user made their first non-anonymous post. This leads to an inaccurate count of the user's threads being saved in the database. In other words, the function doesn't filter out the posts made before the user started posting non-anonymously, causing the stored count to be incorrect.

Note: this fix is not backward compatible as that would require direct changes to the database.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (20cd380) 96.15% compared to head (25f8093) 96.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
+ Coverage   96.15%   96.16%   +0.01%     
==========================================
  Files          58       58              
  Lines        4573     4588      +15     
==========================================
+ Hits         4397     4412      +15     
  Misses        176      176              

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