convert other solr-field backed blacklight configuration fields to serialize as hashes with an enabled flag

remove the blacklight configuration show_fields; use a flag on the
index_fields instead

update the metadata fields edit form to use checkboxes for the different
results views (fixes #69)
Merge pull request #93 from sul-dlss/metadata-fields-admin

convert other solr-field backed blacklight configuration fields to serialize as hashes with an enabled flag