Follow-up from "Import more resources using events in GitHub Import"

The following discussion from !139410 (merged) should be addressed:

• @.luke started a discussion: (+1 comment)

  GitHub Import stats for merged_by, notes, pull_request_review_request and pull_request_review will no longer exist as the resources will be included in the issue_events stats.

  @rodrigo.tomonari In a follow-up issue could we address this? Perhaps we don't count issue_events as a stat any more as it's reasonably meaningless, but instead, we count the different types of issue_events instead? From memory, the frontend is quite out-of-the-box, once the keys are added to the STATISTIC_ITEMS defined in javascripts/import/constants.js (what I noticed in !132196 (merged)).