Remove empty access panels column for image collections.
Enable 'Digital collections' link on home page.
Merge pull request #180 from sul-dlss/image-coll-panels

Remove empty access panels column for image collections.
Merge pull request #181 from sul-dlss/collections-link

Enable 'Digital collections' link on home page.
Add index authors solr document mixin (logic ported from old codebase).
Pass options through SolrDocument#collection_members to CollectionMembers class.
Add collection member enumration helper method.
Add file collection members partial.
Add index and show partials for file collection objects.