Featured Content: Don't remove setting validation.

Erroneously removed in a66a7e9245c4dedef1f655c81e0366df0a40d6a6.

For option-type customizations, sanitization callbacks are registered through `register_setting()`.
To avoid the callback being triggered on the Reading settings page, give it its own option-group parameter.
Merge pull request #736 from obenland/master

Featured Content: Don't remove setting validation.