Decimal points are user preferences, should be grade_item field

For the same reasons that the gradedisplaytype setting was removed from gradebook user preferences, and added to the grade_item table, it would appear that the decimal points should also receive the same treatment.

Rationale:

1. "Real" and "Percentage" display types both use decimal points
2. It makes no sense giving each grade_item control over the gradedisplaytype without also giving control over how these 2 types of display are... displayed

Additionally, constants do not need to be defined for the decimal points, because it would mean defining 12 new constants (0-5 for real and percentage display types). Simple integers will suffice.

It is thus proposed to add a "decimals" field to grade_item (tinyint(1)) and apply the same type of class and form changes that were applied to the gradedisplaytype setting.