UI improvements

* 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.

client/components/boards/boardBody.styl

@@ -0,0 +1,34 @@
+@import 'nib'
+
+.board-wrapper
+  left: 0
+  top: 0
+  bottom: 0
+  right: 0
+  position: absolute
+  overflow: hidden
+
+  .board-canvas
+    position: absolute
+    left: 0
+    right: 0
+    top: 0
+    bottom: 0
+    transition: margin .1s
+
+    &.next-sidebar
+      margin-right: 248px
+
+.lists
+  align-items: flex-start
+  display: flex
+  flex-direction: row
+  margin-bottom: 10px
+  overflow-x: auto
+  overflow-y: hidden
+  padding-bottom: 10px
+  position: absolute
+  top: 0
+  right: 0
+  bottom: 0
+  left: 0