Set ReviewerUserId only if Status changed

For non-admin reviewers (i.e., without `CanReviewOwnArtwork`), the `PATCH` form
has a hidden element with the artwork's current `Status`. If the reviewer
updates the `EbookUrl` and not `Status`, then don't record the reviewer's
`ReviewerUserId` because they didn't review or change the `Status`.

Side note: Sending the `PATCH` form an invalid `Status` will result in a
validation error, but that validation error was hard to read because it was the
wrong exception type. This commit adds a new `InvalidArtworkStatusException` to
fix that.

Fixes #433

lib/Exceptions/InvalidArtworkStatusException.php

@@ -0,0 +1,7 @@
+<?
+namespace Exceptions;
+
+class InvalidArtworkStatusException extends AppException{
+	/** @var string $message */
+	protected $message = 'Invalid artwork status.';
+}