Remove updateChallengeCompletionMetrics scheduled job

[jake-low]

#1236 added a new completion_metrics JSONB column which is kept up to date by triggers, but it did not remove the scheduled job that maintains the old completion_percentage and tasks_remaining counts. Having both at once creates write contention on the challenges and projects tables and makes lots of API operations very slow while the job is running.

This PR does the following:

• drops the completion_percentage and tasks_remaining columns (and computes those fields from the completion_metrics data when building JSON responses)
• removes the updateCompletionMetricsOfActiveChallenges job which maintained those columns
• changes when the update_challenge_completion_metrics and update_project_completion_metrics SQL functions are triggered, so updating a task (or challenge) will only recompute metrics for the parent challenge (or project) if a column was modified that could affect the results.

There are still problems with the present approach which we should fix in a future PR:

• The metrics calculation SQL functions are triggered per row, not per statement. This means that a bulk operation (like inserting 10,000 new tasks) calls the function 10,000 times, when calling it once at the end would be sufficient.
• The code paths for creating a challenge or refreshing tasks build task models one at a time and then INSERT or UPDATE them in individual queries. This is an antipattern on its own (refreshing a challenge with 50k tasks means 50k DB roundtrips), but it also means that even if we switch to per-statement triggers as described above, we still won't see any benefit until these code paths are refactored to insert/update tasks in batches.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[CollinBeczak]

LGTM, i tested locally and things work well in both frontends. Will keep the problems you noted in mind.