Add "you can only enter one value" UI limits to Herald "set status" and "set priority" actions

Summary:
Ref T13048. Fixes T11112. I mostly fixed this before in D18887, but missed that these other actions are also affected. T11112 had a more complete list of missing limits.

(It's possible there are some others somewhere, but this is everything we know about, I think.)

Test Plan: Created rules using these actions, typed stuff into the box, was only able to enter one value.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13048, T11112

Differential Revision: https://secure.phabricator.com/D18943

src/applications/maniphest/herald/ManiphestTaskPriorityHeraldAction.php

@@ -61,7 +61,8 @@ final class ManiphestTaskPriorityHeraldAction
   }
 
   protected function getDatasource() {
-    return new ManiphestTaskPriorityDatasource();
+    return id(new ManiphestTaskPriorityDatasource())
+      ->setLimit(1);
   }
 
   protected function getDatasourceValueMap() {

src/applications/maniphest/herald/ManiphestTaskStatusHeraldAction.php

@@ -58,7 +58,8 @@ final class ManiphestTaskStatusHeraldAction
   }
 
   protected function getDatasource() {
-    return new ManiphestTaskStatusDatasource();
+    return id(new ManiphestTaskStatusDatasource())
+      ->setLimit(1);
   }
 
   protected function getDatasourceValueMap() {