Start the migration from iron-router to flow-router

Motivations:

* Iron-Router foces us to use Tracker.nonreactive black magic in order
  to avoid un-necessary re-renders;
* There is a community consensus (supported by some MDG members) that
  the flow-router API is easier to reason about;
* The useraccounts now supports flow router (that was a blocking
  element when I considered the switch ~3months ago)

On the server we use the Picker router, as encouraged by the Kadira
team (which develop both Flow and Picker routers).

In the current state of things there are some bugs related to the
missing Loading architecure. Previously onRendered callback where
always called when the data the component needed was available, now
we have to handle this ourselves, which we will in a following commit.

client/components/sidebar/sidebar.js

@@ -162,7 +162,7 @@ Template.labelsWidget.events({
 // fields of the current board document.
 var draggableMembersLabelsWidgets = function() {
   var self = this;
-  if (! Meteor.userId() || ! Meteor.user().isBoardMember())
+  if (! Meteor.user() || ! Meteor.user().isBoardMember())
     return;
 
   self.autorun(function() {

client/components/sidebar/sidebarArchives.jade

@@ -10,7 +10,7 @@ template(name="archivesSidebar")
         | -
         a.js-delete-card Delete
       if cardIsInArchivedList
-        p.quiet.small (warning: this card is in an archived list) <br>
+        p.quiet.small (warning: this card is in an archived list)
     else
       p.no-items-message No archived cards.