Improve scrolling

We now replace native scrollbar by custom ones on the list card (which
is required by the new ergonomics in the parent commit), but the
"scrolling engine", is still native, we just hide the scrollbar and
draw our own in HTML/CSS using the perfect-scrollbar package (from
bower).

This commit also implements component scrolling when certain actions
are performed, eg scroll to the bottom when the new card composer is
opened.

client/components/mixins/infiniteScrolling.js

@@ -0,0 +1,37 @@
+var peakAnticipation = 200;
+
+Mixins.InfiniteScrolling = BlazeComponent.extendComponent({
+  onCreated: function() {
+    this._nextPeak = Infinity;
+  },
+
+  setNextPeak: function(v) {
+    this._nextPeak = v;
+  },
+
+  getNextPeak: function() {
+    return this._nextPeak;
+  },
+
+  resetNextPeak: function() {
+    this._nextPeak = Infinity;
+  },
+
+  // To be overwritten by consumers of this mixin
+  reachNextPeak: function() {
+
+  },
+
+  events: function() {
+    return [{
+      scroll: function(evt) {
+        var domElement = evt.currentTarget;
+        var altitude = domElement.scrollTop + domElement.offsetHeight;
+        altitude += peakAnticipation;
+        if (altitude >= this.callFirstWith(null, 'getNextPeak')) {
+          this.callFirstWith(null, 'reachNextPeak');
+        }
+      }
+    }];
+  }
+});