Add "mailbox" tag for Postman Beware (C&D)

When your sim reaches Level 2 of the Veterinarian career in Cats & Dogs, they unlock two mailboxes - a cat design (Meow Mailbox) and a dog design (Postman Beware).

Searching for "mailbox" pulls up Meow Mailbox and other base game mailboxes, but not Postman Beware.  I'm guessing it's because the name doesn't contain the word "mailbox," but it would be helpful if it still came up when searching.