Facilitate offline migrations for large tables with helper

[agritheory]

• Check number of rows with threshold set in hooks.py for all doctypes
• use before_migrate hooks to detect what tables will change; if any of those meet the threshold, prompt user to abort the migrate and run offline migrate command
• Use percona-toolkit and pt-online-schema-change to migrate changes in tables. Because we have already detected the size of the tables that meet the threshold, we can programmatically interface with pt-online-schema-change