We didn't have any support for actually changing a DecimalField's

max_digits and decimal_places attributes, which meant that once such

a field was added to a model, it could never be changed.
This change implements the ability to change these attributes. It

requires a bit of a change to the column attribute mutation logic, since

unlike most attributes, max_digits and decimal_places really need to

be considered together when updating the column. That's because in most

databases, they're defined as part of the column type

(numeric(max_digits, decimal_places)). The approach implemented is a

bit specialized for DecimalField, but if we encounter more situations

like this in the future, we'll be able to build upon it.

Unit tests pass for all databases and all versions of Django.