CAS-1360: Implementation of the REST API with Spring @RestController

Contains all the changes from the review of earlier PR and also the GH pages REST documentation is updated with a pending PR. No unit tests yet (although tested fully manually locally), but could work on that and create a separate PR. Please note that the original restlet module does not contain unitit tests, so I personally think that this PR/module does not 'violate' it in this regard ;-)

Best,
D.