Explorer view: Handling preprints

[jvwong]

Short term fix for Explorer, specifically for preprints;

• Since the related papers system is tied to the use of PMIDs, with a preprint, these will simply not be shown on the main 'document' page.
• Google Scholar link uses title as query (instead of DOI) to cover edge case where some preprint DOIs aren't indexed

Case: Preprint

Before

Case: No article found

Case: No abstract

Refs #1215

[maxkfranz]

Looks great