[Users] Implement undo user delete feature

Once https://github.com/coding-blocks/gondor/issues/134 is resolved, we need a way to undo the delete by the admins.

- [ ] add a `userEnable` mutation to undo the delete.
- [ ] allowed only for admins.
- [ ] add an argument to the users query type named 'includeDisabled' based on which you include the disabled entities in the results too. By default it can be false. Even if it is true, the disabled entities shall only be included if the user is admin.
- [ ] in the users query type, the default order should be based on the deleted_at, and should also be used as a fallback even if another column is provided via args to sort based on.