DB API returns JSON-API compatible objects. Export triggers 'Save As' dialog.

closes #2647
- GET method returns { db: [exportedData] }
- POST, DELETE methods return { db: [] }
- 'delete all content' test updated
- Attach 'Content-Disposition' header on DB export for 'Save As' browser dialog
- Add DB API functional test for Export
Merge pull request #2706 from appleYaks/db-api-update

DB API returns JSON-API compatible objects. Export triggers 'Save As' di...