Expose a way to create/initialise the default board through the API

Description

There is currently no way to create an issue board on a project with the API. While this is fine since there's only one board per project on CE, it's a problem when automating lists creation on new projects since the board is only created when clicking on "add default list" or "no I'll add my own".

Proposal

A create/initialize board endpoint would be the easiest. Making the board_id on the other endpoint seems impractical. Creating the board on the first call to /boards might do the trick but that doesn't feel too clean.

Links / references

The boards API: https://docs.gitlab.com/ce/api/boards.html

Use cases

This feature is needed for those who wish to automate the project's structure and need to add custom lists to the project's issue board.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.