CMS-1741 Add type definitions for relationshipType_get in RemoteService.ts

Add strict typing for the parameter and result values of RemoteServiceInterface.relationshipType_get()

* Check the RpcHandler method in Java to find the exact values accepted and returned.
* Use optional types (optParam?: string) where appropriate.
* Avoid use of any as much as possible and use concrete types.
* Look at types of space_list() and space_get() for reference.
Merge branch 'master' of github.com:enonic/wem-ce into CMS-1741

Conflicts:
	modules/wem-webapp/src/main/webapp/admin2/api/js/lib/RemoteService.ts
Merge pull request #268 from enonic/CMS-1741

CMS-1741 Add type definitions for relationshipType_get in RemoteService....