Add indexes to Dossier.state, Dossier.archived, Follow.unfollowed_at

We make a ton of queries on these attributes (for example in /procedures#index and /procedures#show). I think it should help.

db/migrate/20190920122228_add_indexes_to_dossier.rb

@@ -0,0 +1,7 @@
+class AddIndexesToDossier < ActiveRecord::Migration[5.2]
+  def change
+    add_index :dossiers, :state
+    add_index :dossiers, :archived
+    add_index :follows, :unfollowed_at
+  end
+end