Fixed #21730 -- Removed superfluous db_index in custom auth docs

Thanks rafalp for the report.
Refactored DatabaseSchemaEditor to allow 3rd party hooks.

3rd party backends need more control over the SQL generated for schema
migrations without being forced to override most of the class methods.
Provide the old column type definition to _alter_db_column_sql to help backends that need to copy columns
Merge branch 'schema-editor' of https://github.com/manfre/django into schema-editor