Contact Form: Introduce checkbox-multiple field type.

The checkbox type only allows a single checkbox per field, but this new one will allow multiple checkboxes to be selected.

Note: e3c170ab9d2d571d6ee4ba9c65dd3eea340ae5e5 mentions two bugs in the initial commit; the second bug was fixed in f8ac1a5, and the first turned out to be in master itself, rather than something that e3c170ab9d2d571d6ee4ba9c65dd3eea340ae5e5 introduced (see 9d9e854a2d581e0f7dccfde6b136ffecb47d4b70, 5064129e632717471bbf3486c59527b244e215c2, and 10f36b2977aeb4b778d9934cfa2dbab5ddf792f2).