Migrations: Update `create_migrator.py` to use template file and omit leading/trailing newlines

[eecavanna]

In this branch, I made two changes:

• The create_migrator.py script no longer puts a leading newline and extra trailing newline into the migrator it creates
• The migrator template is now defined in a dedicated file, instead of in a string variable embedded within that script

I have tested the changes locally and am ready for them to be reviewed/merged.