Replace the old (and broken) jshint + jscsrc by eslint and configure
it to support some of the ES6 features.
The command `eslint` currently has one error which is a bug that was
discovered by its static analysis and should be fixed (usage of a
dead object).
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.
