Implemented Add Permission modal and controller

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

gms-ui/src/components/Main.vue

gms-ui/src/components/modals/AddMemberModal.vue

gms-ui/src/components/modals/AddPermissionModal.vue

gms-ui/src/components/modals/SearchUser.vue

gms-ui/src/store.js

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

...ava/it/inaf/ia2/gms/controller/PermissionsController.java

...rc/main/java/it/inaf/ia2/gms/model/PermissionRequest.java

...main/java/it/inaf/ia2/gms/service/GroupsModelBuilder.java

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

...main/java/it/inaf/ia2/gms/service/PermissionsService.java

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

...af/ia2/gms/service/PermissionsServiceIntegrationTest.java