GraphQL: Add mutation to create the default backlog list for a board

We recently added support in GraphQL to query for the lists of a board. In doing so, we made a conscious decision not the automatically create the backlog list if it didn't exist.

We now need to add a mutation that allows us to create that list.

It would most likely be part of an implementation of https://docs.gitlab.com/ee/api/boards.html#new-board-list