Simplified _get_issue_urls by calling git notes instead of show-ref
Fix issue URLs for-loop to include the last page

Closes #8 (along with defaulting to 1 in c3621be).
Dual-purpose _get_all_urls to work for issues and for comments
Switch notes name to "issues" in order to match "comments" note
Don't need to export GH_URL; not being used in any sub-processes
Added printf-style constant for the issues URL path
Added helper functions to retrieve issues & comments URL lists
Updated sync() function to call _process_issues() directly