Move trigger of `i_code_review_user_single_file_diffs_monthly` to new endpoint used for single-file file-by-file UX

All event reporting for the usage of single file mode has dropped off.

After a bit of digging, it appears to me that the only time this event is triggered is on this line.

• (that method then calls here, which limits the event reporting to when the user has this feature enabled)

However, as a result of intentionally switching away from loading all of the files at once - especially and critically for single-file mode - the endpoint this is reported on (:diffs_batch) is no longer called. The single-file file-by-file mode now uses diff_for_path. diff_for_path is also used to expand collapsed files in full diffs (e.g. not in file-by-file mode), so it's not exclusive to file-by-file.

Problem

i_code_review_user_single_file_diffs_monthly is no longer being reported, because it was attached to the usage of the diffs_batch endpoint.

Solution

Options:

1. Report i_code_review_user_single_file_diffs_monthly any time files are served when the setting is turned on for the user, regardless of what endpoint is used
2. Report i_code_review_user_single_file_diffs_monthly when the diff_for_path endpoint is used and the view_diffs_file_by_file setting is enabled for the user.