oc: Honor calendar WebMail UI permissions on OpenChange library

By expanding roles from the given ACL to have these values as flags
inside the OpenChange library. This only applies to Calendar and
Tasks folders which stored four different access rights to three
different types of events/tasks.

As the events and tasks are stored in the same table, I have added
two new classes to manage permissions in the same way and this
avoids the code duplication called MAPIStoreCalTask(Folder|Message).

OpenChange/MAPIStoreMessage.m

@@ -981,12 +981,22 @@ rtf2html (NSData *compressedRTF)
       activeUserRoles = [[context activeUser]
                           rolesForObject: sogoObject
                                inContext: [userContext woContext]];
+      /* We use in this library the roles as flags, so we expand high
+         access rights with the lower ones */
+      activeUserRoles = [self expandRoles: activeUserRoles];
       [activeUserRoles retain];
     }
 
   return activeUserRoles;
 }
 
+/* Expand current roles with lower access roles to transform them to
+   flags */
+- (NSArray *) expandRoles: (NSArray *) roles
+{
+  return roles;
+}
+
 /* Can the current active user read the message? */
 - (BOOL) subscriberCanReadMessage
 {