Remove tracker role

The current tracking section had a few issues:

* When browsing as an admin, this section becomes useless since no
investments are shown
* Browsing investments in the admin section, you're suddenly redirected
to the tracking section, making navigation confusing
* One test related to the officing dashboard failed due to these changes
and had been commented
* Several views and controller methods were copied from other sections,
leading to duplication and making the code harder to maintain
* Tracking routes were defined for proposals and legislation processes,
but in the tracking section only investments were shown
* Probably many more things, since these issues were detected after only
an hour reviewing and testing the code

So we're removing this untested section before releasing version 1.1. We
might add it back afterwards.

app/controllers/admin/legislation/milestones_controller.rb

@@ -1,4 +1,4 @@
-class Admin::Legislation::MilestonesController < Tracking::MilestonesController
+class Admin::Legislation::MilestonesController < Admin::MilestonesController
   include FeatureFlags
   feature_flag :legislation
 
@@ -11,4 +11,8 @@ class Admin::Legislation::MilestonesController < Tracking::MilestonesController
     def milestoneable
       ::Legislation::Process.find(params[:process_id])
     end
+
+    def milestoneable_path
+      admin_legislation_process_milestones_path(milestoneable)
+    end
 end