Custom sort value field

I prefer using a different field name for the sort field.  I actually migrated from a different sortable app and didn't want to change the field name.

This adds a `SORTEDM2M_FIELD_NAME` setting which allows the field name to be overridden.  I also added an escape for the field name when querying so reserved names like "order" can be used.

I intended to add tests for this feature before creating a pull request but I never got around to it.  I'm creating this pull request to at least start the discussion about this feature.