Bug 20004: Adapt /v1/cities to new naming guidelines

Bug 20004: Adapt /v1/cities to new naming guidelines

This patch implements the changes required by the cities endpoint RFC
[1].

It uses the objects.search helper, and relies on bug 19686.

To test:
- Apply the patches
- Compare the spec with the RFC (api/v1/swagger/definitions/city.json)
=> SUCCESS: It makes sense
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/cities.t
=> Tests pass!
- Sign off :-D

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>