Allow user to search in classifications

Refs https://redmine.teklia.com/issues/9604

Depends backend#1881 (closed)

A search user should be able to trigger the new classification source:

• add a new Classification source item on the list below the search query
• the checkbox must be enabled by default
• the API call must send that new sources amongst others

The display already list classes when present on results

changed milestone to %Arkindex 1.7.1

added Feature P2 labels

assigned to @mlbonhomme

mentioned in merge request !1816 (merged)

closed with merge request !1816 (merged)