Change single-user lookups by ID in the web-gateway to use the UserService

[TimMoore]

There are a few places in web-gateway that look up a user by looping through nav.getUsers() and checking for a matching user ID. The current user is also looked up from the same list, and passed when creating a new Nav

With the changes in #131, this list of users is incomplete and eventually consistent, so this won't always work.

The following places in the code should be changed to stop using nav.getUsers(), and instead use userService.getUser(userId) to look up the user instead:

• ItemController.doGetItem
• TransactionController.getTransaction
• AbstractController.loadNav (here, it isn't calling nav.getUsers(), because it hasn't created nav yet, but it is doing something similar: looping through the list of users returned from userService.getUsers to look for the current user ID, which may not be in that list)

Note that, because service calls are asynchronous, you'll now need to use the CompletionStage API. You might find it helpful to refactor the existing controller methods to extract small parts into helper methods.

At the end of this change, the only thing left using nav.getUsers() should be the list of users that displays in the navigation menu.

This should also fix #144. Please be sure to test manually to verify this.

[TimMoore]

Fixed by #143