The app signature was storing the starting and last applied migration

names, but as migrations are a graph, this wasn't sufficient. We're not

guaranteed any order when it comes to migration names (the numeric

prefix is just a convention), and things like squashed migrations make

this more complicated.
Now, the signature stores the complete list of migrations that have been

applied to an app, removing the old data on starting and last applied.
The MoveToDjangoMigrations mutation takes this as well, as the only

parameter. It defaults to 0001_initial, but if the existing evolution

history for an app is equivalent to more than one migration, those can

be specified directly when constructing the mutation.

Unit tests pass on all supported versions of Django.