The UI and the internal APIs are still rough around the edges but the
feature is basically working. You can now select multiple cards and
move them together or (un|)assign them a label.
* Implement visibility choice on board creation;
* Rework the board header bar. Remove links to un-implemented
features;
* Implement a board star counter (visible if the board have >2 stars);
* Define a new icon (a thin cross) to close elements;
* Remove $(document).on('mouseover') event handlers that were
basically fired hundreds of times for nothing, we now define a proper
Tracker dependency to execute jquery-ui plugin initialization only
when something has changed;
* Bug fixes related to list scrolling.
