Add Selected Databases access point.
Enable selected databases link on home page and databases access point masthead.
Abstract generic link logic into its own class.
Add index links mixin and include in SolrDocument.
Only show MARC 592 for databases.
Merge pull request #122 from sul-dlss/selected-databases

Add selected databases access point
Merge pull request #124 from sul-dlss/db-marc592

Only show MARC 592 for databases.
[WIP] Add Google Book covers to search results and record view