Restore placeholders in issue filters from "Any" to "Milestone" and "Label"

1f11096c included new filters but made it hard to determine which dropdown did
what. This patch restores the original placeholders.

### Before

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/2326b836e364275341ea0ad8c0a0926b/image.png)

### After

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/b0e519f9cf8d1a3c0f054286e3b809cb/image.png)


See merge request !1554