Add support for an arbitrary number of boards on the Gameboard

[rbclark]

(Requested by ECTF team)

Currently we only have 2 possible gameboards that can be displayed on the homepage at a time. The ECTF team would like to be able to have 3 boards display

Design Phase Challenges
Optional Challenges
Attack Defense Challenges

It makes the most sense to me that we should just add a concept of boards where the game has_many :boards and each board has_many :challenges and then we can allow users to set a title for the board, etc. This also means we can get rid of the hack that I currently have in the board to display attack/defense and design phase challenges at the same time.