Fix empty 'Available online' panel
Merge pull request #175 from sul-dlss/online-panel-fix-take-2

Fix empty 'Available online' panel
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 'In collection' access panel to metadata panels section.

Add druid mixin to extract the druid reliably from any purls in index URLs.