R1:a46c25d2baf3

Make two ancient migrations fatal if they affect data

Summary:
Depends on D20106. Ref T6703. Since I plan to change the `ExternalAccount` table, these migrations (which rely on `save()`) will stop working.

They could be rewritten to use raw queries, but I suspect few or no installs are affected. At least for now, just make them safe: if they would affect data, fatal and tell the user to perform a more gradual upgrade.

Also remove an `ALTER IGNORE TABLE` (this syntax was removed at some point) and fix a…