Create method to keep `m_backups` clean

[jamesmontalvo3]

Create method to keep directory clean like:

• Only keep one backup from the last day (the newest?)
• Only keep one backup between yesterday and a week ago (the oldest?)
• Only keep one backup between a week ago and a month ago (the oldest?)
• Only keep one backup between a month ago and two months ago (the oldest?)
• ...and so on...

[jamesmontalvo3]

What about:

• Keep latest backup .sql file
• Compress all previous:
  • Keep first backup .sql file
  • Create incremental backups (diffs against first) of all subsequent
  • gzip first + incrementals into a <wiki_id>_backup_YMDHis_to_YMDHis.tar.gz