Bug 17314: Implement /suggestions routes

Bug 17314: Implement /suggestions routes

This patch introduces routes to handle purchase suggestions, from the
staff POV.

Tests are added as well.

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/suggestions.t
=> SUCCESS: Tests pass! And they are meaningful!
3. Play with your favourite REST tool (Postman?)
4. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>