tags: REST API addition

* Creates API for tags, in order to have a reusable API
  both for restful API and views probably.

* Refactors tags REST API in order to use the new tag API.

* Tests the tags REST API.

* Re-orders import statements.

* Adds init file in the testsuite sub-package.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>