This introduces support for all database types on Django 1.11. The major

differences in this version have to do with how index names are

generated (again), how Postgres constraint-related SQL is generated

(again), and the introduction of yet another way of setting indexes on a

model.
Django 1.11 allows explict indexes to be defined by setting a model's

Meta.indexes to a list of models.Index instances, which can span

fields. This change introduces support for representing this in the

signature, to set new indexes in ChangeMeta, and to evolve these

changes.
It also handles the other compatibility issues for index names (which,

unfortunately, are not consistent between the new models.Index

instances and the older indexes) and changes to generated SQL.

Unit tests pass for all supported database types on Django 1.6 through

Django 1.11.