Fixed #23941 -- Aggregating over decimal field regression

This patch fixes the regression but probably isn't ready for commit just yet. 

What this patch does is ignores the "max_digits" and "decimal_places" arguments of a DecimalField for all annotations. Setting your own arbitrary digits/decimals makes no sense with regards to aggregation, but it may make sense for other annotations.
This will now only attempt to format the decimal if it has precision information available. We could, instead, change the typecast_decimal function to handle missing digits/places, but I'm not sure that'd be entirely appropriate.