record upkeep year and use it for filtering checklist

[thomasp85]

Fix #1916

This PR adds two new functions: record_upkeep_year() which writes the year of the upkeep to Config/usethis/upkeep and last_upkeep_year() which retrieves this value or defaults to 2000 if it doesn't exist. These two functions are used in use_tidy_upkeep_issue() to automatically use the year of the last upkeep for filtering the produced checklist as well as incrementing the recorded year to the current year.

Further, this PR fixes what appears to be a bug/discrepancy in how year was used in make_upkeep_issue() depending on whether use_upkeep_issue() or use_tidy_upkeep_issue() was called