Associate repos and/or issues with boards

[chillu]

The way I understand the Zenhub data model:

• A board is associated to one repo
• A board can have show issues from many repos
• An issue can be on one board only (this might change)

The Zenhub Boards API only allows listing issues on the board for a specific repo, not discovering which boards actually exist. We could add a "board repo" column to each issue which can be separate from the repo the issue is actually on

[chillu]

Filed an issue on doing this better through their API at https://github.com/ZenHubIO/API/issues/105