Add a Sidebar class to represent the side bar

Start using it throughout the rest of the app instead of the various
instance variables

app/models/sidebar.rb

@@ -0,0 +1,29 @@
+class Sidebar
+  attr_reader :contexts, :projects
+
+  def initialize(user)
+    user = user
+    @contexts = user.contexts
+    @projects = user.projects
+  end
+
+  def active_contexts
+    @active_contexts ||= contexts.active
+  end
+
+  def hidden_contexts
+    @hidden_contexts ||= contexts.hidden
+  end
+
+  def active_projects
+    @active_projects ||= projects.active
+  end
+
+  def hidden_projects
+    @hidden_projects ||= projects.hidden
+  end
+
+  def completed_projects
+    @completed_projects ||= projects.completed
+  end
+end