Watching: Prevent issues when watchable or user is deleted

- Adds filtering to the watched items list in notification preferences
  so that deleted (recycle bin) items are removed via query.
- Adds relations and logic to properly remove watches upon user and
  entity delete events, to old watches in database do not linger.
- Adds testing to cover the above.

Did not add migration for existing data, since patch will be close to
introduction, and lingering DB entries don't open a security concern,
just some potential confusion in specific potential scenarios.
Probably not work extra migration risk, although could add in future if
concerns/issues are found.

Related to #4499

tests/User/UserPreferencesTest.php

@@ -124,6 +124,23 @@ class UserPreferencesTest extends TestCase
         $resp->assertDontSee('All Page Updates & Comments');
     }
 
+    public function test_notification_preferences_dont_error_on_deleted_items()
+    {
+        $editor = $this->users->editor();
+        $book = $this->entities->book();
+
+        $options = new UserEntityWatchOptions($editor, $book);
+        $options->updateLevelByValue(WatchLevels::COMMENTS);
+
+        $this->actingAs($editor)->delete($book->getUrl());
+        $book->refresh();
+        $this->assertNotNull($book->deleted_at);
+
+        $resp = $this->actingAs($editor)->get('/preferences/notifications');
+        $resp->assertOk();
+        $resp->assertDontSee($book->name);
+    }
+
     public function test_notification_preferences_not_accessible_to_guest()
     {
         $this->setSettings(['app-public' => 'true']);