Add UserController for API v3

src/main/java/de/thm/arsnova/controller/UserController.java

+package de.thm.arsnova.controller;
+
+import de.thm.arsnova.entities.LoginCredentials;
+import de.thm.arsnova.entities.UserProfile;
+import de.thm.arsnova.services.UserService;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RestController
+@RequestMapping("/user")
+public class UserController extends AbstractEntityController<UserProfile> {
+	private UserService userService;
+
+	public UserController(final UserService userService) {
+		super(userService);
+		this.userService = userService;
+	}
+
+	@PostMapping("/register")
+	public void register(@RequestBody LoginCredentials loginCredentials) {
+		userService.create(loginCredentials.getLoginId(), loginCredentials.getPassword());
+	}
+}