Refactor serialized types to be partially defined as custom properties

Many of the methods defined in `AttributeMethods::Serialization` can be
refactored onto this type as well, but this is a reasonable small step.
Removes the `Type` class, and the need for `decorate_columns` to handle
serialized types.
Merge pull request #15398 from sgrif/sg-serialized-types

Refactor serialized types to be partially defined as custom properties
Fix broken link in Rails 3.0 release_notes
Merge pull request #15411 from nathany/patch-1

Fix broken link in Rails 3.0 release_notes [ci skip]
Add missing period from 4b802bc [ci skip]