Data migration for BuildConfig model (#12698)
This is the continuation of #12646
---
We have a lot of builds and we need to iterate over them to copy the
data from `Build` model to `BuildConfig`. I'm a little worried about how
much time this migration could take. We can probably perform this
migration _outside_ a deploy by copying and pasting the code into a
Django shell --maybe with some edits to wait a 100ms after 1k iterations
or similar. What do you think?
```
In [2]: Build.objects.filter(_config__isnull=False).count()
Out[2]: 22680759
```
