Enable or disable spell checking web service

Setting the policy to Enabled puts a Google web service in use to help resolve spelling errors. This policy only controls the use of the online service. Setting the policy to Disabled means this service is never used. Leaving the policy unset lets users choose whether to use the spellcheck service. The spell check can always use a downloaded dictionary locally unless the feature is disabled by SpellcheckEnabled in which case this policy will have no effect.