Allow creation of a MigrationList from a list of migration names.
Review Request #11222 — Created Oct. 14, 2020 and updated
MigrationList.from_names()allows for easy creation of a
MigrationListfrom just an app label and a set of migration names,
without any need to inspect the database or a signature. This will
be used in upcoming changes that need to more carefully build and
operate on lists of migrations.
Unit tests pass.