Apparently, our Python 3 conversion for the early-migrations logic
here was incorrect, and as a result we never set
need_create_large_indexes to True (because we were checking whether a
`bytes` was inside a list of `str`s).
The simplest fix would be to just add a `.decode()` in one place, but
this refactor to just decode at the beginning is a lot more readable.
b564976ce4