Add account export

TO DO

• Config: days to keep and export limit in days
• Add backups table
• API: List Archives
• API: Request a new archive
• Add API documentation
• Email: Archive is ready to download
• Refuse backups if instance doesn't have email enabled
• Refuse backups if user doesn't have an email address
• Admins should be able to generate backups on behalf of any user without limitation on how many per time period
• Expired archives cleanup
• Update CHANGELOG

Closes #847 (closed)