feat(user-sessions): implement user login, logout, and session management

knockoutwhistweb/app/logic/user/UserManager.scala

@@ -1,10 +1,15 @@
 package logic.user
 
+import com.google.inject.ImplementedBy
+import logic.user.impl.StubUserManager
+import model.users.User
+
+@ImplementedBy(classOf[StubUserManager])
 trait UserManager {
 
   def addUser(name: String, password: String): Boolean
-  def authenticate(name: String, password: String): Boolean
-  def userExists(name: String): Boolean
+  def authenticate(name: String, password: String): Option[User]
+  def userExists(name: String): Option[User]
   def removeUser(name: String): Boolean
 
 }