Search work, all specs passing.

Search by:

- Freetext
- country
  + ISO2 code
  + eldis id
- region
  + UN code
  + eldis id
- theme
  + eldis id
  + agrovoc id (slug)
- fix next/previous links in metadata etc..

TODO
  - consider supporting search by name.  Implementing it can be done,
    but it has some problematic semantics... i.e. there's no way to tell
    a name query apart from an agrovoc id, so the best you can do is a
    union in that case; but that might give more results than you want
    as it will match on titles.