BibKnowledge: Additional fixes to json export API
- Adds two API functions for use in, e.g., BFEs, etc: + bibknowledge.get_kbd_values_json() + bibknowledge.get_kb_mappings_json()
- /kb/export REST API supports JSON output with format types 'jquery' OR 'json'.
- Separation of unit and regression tests
- New regression tests
- Refactoring inside bibknowledgeadmin.kb_export to make for easier reading
(closes #604)