Merge ref module with missing-redirects in new stat module

[spotman]

• Use stat_pages table for target urls (existing and missing, distinguish by flag is_exists)
• Use stat_hits table for registering existing and missing hits (same flag)
• Extract missing_url_referrers logic into stat_links and stat_page
• Update stat_hits processing cron task
• Use stat_pages.redirectTo field for creating redirect from one mising stat_page to another existing
• Remove redirect_targets, missing_url_referrers
• Deal with missing_urls_missing_url_referrers table (use ref_links instead)
• Update notification logic for missing urls
• Allow to select redirect status code for missing urls (301, 302, 410 and 404 as default if no redirect target set)

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https://habrahabr.ru/company/cloud4y/blog/349028/