Notification icons: missing icon for custom

In !40709 (merged) we update the notification icons to use our internal Gitlab SVGs.

However, when a user selects the Custom notification setting, the sprite_icon helper responds with the following error as it cannot find an icon for the custom level:

is not a known icon in @gitlab-org/gitlab-svg

We either need an entry/icon for custom or at least a default state/icon.