Add field to DataField and DataConcept for excluding from search index

Fix #172.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
This change requires a data migration as well since the non-enumeral/searchable fields will now be indexable since the default is true above. Or maybe the default should be false? That would still require a data migration, but I feel that would be the lesser of the two evils. I don't want to index numbers.