Cleanup AlleleSource and old manual migrations

I have replaced all AlleleSource objects - but won’t delete the models from the database yet - as they are needed to run historical manual migrations etc

We need a "checkpoint" to stop and make sure that people have run all outstanding manual migrations before proceeding with normal migrations

Perhaps this "ensure outstanding completed checkpoint" could be added to "manual"? Pretty easy to find them:

```
from manual.models.manual_migration_models import ManualMigrationOutstanding
ManualMigrationOutstanding.outstanding_tasks
```

Just need to print what commit it is, could commit then go back and change?

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After that point, we can delete AlleleSource records

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Also need to remove fields:

AlleleSource and FKs to it
AlleleLiftover - variant_allele - haven’t used this yet - will we ever? We can find out the last successful AlleleLiftover for the allele easily enough
Remove “error_message” (been replaced by error)

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Also get rid of the "one_off_manual" etc etc