sg: Add `sg migration revert <commit>`

Created by: efritz

Add a command to revert all of the migrations defined in a commit that was since reverted from main. This command:

• identifies the set of migrations new to that commit
• replaces their "up" query with a no-op, but leaves their down query alone
• adds a new migration that runs the "down" queries of each migration to ensure that any environments that ran this will have it undone