Fix bug that causes alerts to affect other courses

Previously, `alerts_cache` is modified when `evaluate_for_course` is called. If the course being evaluated has alerts, they will be incorrectly added to `alerts_cache` for that course's account. All subsequent courses in the same account will see them as account alerts when evaluated, even though they were only meant for that one course.

Test Plan:
  * Create two courses in the same account
  * Set up an alert in the first course
  * Create the circumstances that meet the criterion in both courses
  * Confirm that the alert is only triggered in the first course

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*NOTE: We have a signed CLA for all of Simon Fraser University (SFU) on file.*