[1.8.x] Fixed #24554 -- Sped up migrations by rendering initial apps when they are first needed

Calling Migration.mutate_state() now also allows to do in_place
mutations in case an intermediate state is thrown away later.

Thanks Anssi Kääriäinen for the idea, Ryan Hall for parts of the patch,
and Claude Paroz and Tim Graham for the review

Backport of 57dc8dd3 from master