Generic search improvements: added checkboxes for filtering on users or groups

gms-ui/src/api/server/index.js

gms-ui/src/components/GenericSearchResults.vue

gms-ui/src/components/TopMenu.vue

gms-ui/src/store.js

...ain/java/it/inaf/ia2/gms/controller/SearchController.java

...a/it/inaf/ia2/gms/model/request/GenericSearchRequest.java

.../it/inaf/ia2/gms/model/request/PaginatedModelRequest.java

gms/src/main/java/it/inaf/ia2/gms/service/SearchService.java

...java/it/inaf/ia2/gms/controller/SearchControllerTest.java

.../test/java/it/inaf/ia2/gms/service/SearchServiceTest.java