New boardlist route and slight reorganization

In preparation for an upcoming UI reorganization that will eliminate the board list dropdown, this PR introduces a new board list route, view, and controller. This view is not styled, nor do any links point to it at this point, but I thought it was a nice self-contained and bite-sized PR rather than throwing it into a larger future UI reorganization PR.

Sorry about `jia/jia/static/app/editboard/editboard.js -> jia/jia/static/app/board/board.js`. Git realized it was a rename, not sure why Github is showing it as a deletion and a new file.