Move the validator for default values

Even though this validator was just added in 7c144e9, it really belongs
in the `xmas.validators` module. It's being moved there with the
validators that already existed there.

The validators are also being aliased just as WTForms does.
Add validator tests

These tests cover the `DefaultValue` and `Slugify` validators. The
`Unique` validator requires a database connection (or some stubs). This
commit is focused primarily on making sure `DefaultValue` is working
correctly, so tests for `Unique` will come at a later time.