Implement multi-selection

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.

client/components/sidebar/sidebar.styl

@@ -7,7 +7,7 @@
   right: 0
 
   .sidebar-content
-    padding: 10px 20px
+    padding: 12px
     background: white
     box-shadow: -10px 0px 5px -10px darken(white, 30%)
     z-index: 10
@@ -23,7 +23,33 @@
       color: darken(white, 50%)
 
     hr
-      margin: 8px 0
+      margin: 13px 0
+
+    ul.sidebar-list
+      display: flex
+      flex-direction: column
+
+      li a
+        display: flex
+        height: 30px
+        margin: 0
+        padding: 4px
+        border-radius: 3px
+        align-items: center
+
+        &:hover
+          &, i, .quiet
+            color white
+
+        .member, .card-label
+          margin-right: 7px
+
+        .sidebar-list-item-description
+          flex: 1
+          overflow: ellipsis
+
+        .fa.fa-check
+          margin: 0 4px
 
 .board-sidebar
   width: 248px