Consolidate Study and Screening database models

[bdewilde]

changes

• moves Citation and Fulltext database records directly into parent Study records as nested mappings
• consolidates CitationScreening and FulltextScreening records into a single collection of Screening records, using study_id and stage fields instead of citation_id/fulltext_id
• updates db orm relationships and trigger events accordingly
• updates API logic accordingly, without affecting behavior from the perspective of API callers via hacks, alternate schemas, and white lies
• refactors db model imports to module-level, for cleaner namespaces and better behavior in case of future db model changes