Full screen/focus mode Issue Boards

Lets have a fullscreen experience / focus mode for our issue boards!

usecases:

• displaying on external non active screens. perhaps too or like cycle analytics
• just more space/focus

proposal

with or without filters (although with filters is more functional imo)

expand button could be either in filters bar or the current board title bar

Really rough mocked mockups

cc: @iamphill @tauriedavis @awhildy @victorwu