Proposed fixes for sitemap.php admin errors

1. Explicitly set the action parameter for the form.
2. WT_Filter::postBool returns null if the posted variable doesn't exist
(maybe use FILTER_NULL_ON_FAILURE flag?) which is the case if the
checkbox is unchecked. This then means that WT::Tree->preference does a
get not a set.
3. When setting a gedcom preference, although the database is updated
the cached value isn't