Adding missing GroupConcat features issue #1101

[smilerightnow]

Hello, I added the missing feature of issue #1101. Now it's ordering by multiple columns.

I also added two tests, all the tests are green.

[adamchainz]

Hi, thanks for the initial work here.

I think we should make these changes:

1. Don't add a new ordering parameter. Instead, let's update ordering to accept a sequence (list or tuple) of strings representing field names. Then users can specify e.g. ordering=("name",) to order by the name column.
2. Use field names instead of column names, like QuerySet.order_by. You then need to use the model field to map to the column name, and escape the column name with the internal function connection.ops.quote_name.
3. Don't use "asc"/"desc" terminology with field names. Instead, default to asc ordering and support a - prefix for desc ordering, like QuerySet.order_by.
4. Update the docs.
5. Add a changelog note.

Thanks!