Improve import / export functionnality

It is not finished yet and does not even work at all!!
- ZIP archive can be uploaded
- Entries are imported from starred.json and feed*.json but not added in DB for
  the moment.
- Fix export (add author, id -> guid, content -> content.content and add
  alternate)

See https://github.com/marienfressinaud/FreshRSS/issues/163