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).
New features:
- select all filtered cards
- assign or unassign a member to selected cards
- archive selected cards
This commit also fix the card sort indexes calculation when a multi-
selection is drag-dropped.
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.
