Create reports

This table will store which reports (stats, results, ...) will be shown
for a certain process (polls, budgets, ...).

Note Rails fails to save a poll and its report when both are new records
if we add a `validate :process, presence: true` rule. Since it caused a
lot of trouble when creating records for tests during factories rule
completely. Instead, I've created the `results_enabled=` and
`stats_enabled=` methods, so tests are easier to set up, while also
automatically creating a report if it doesn't already exist. This also
decouples form structure and database implemenation.

Originally I named this table `enabled_reports` and instead of having
`stats` and `results` columns, it had an `enabled` column and a `kind`
column, which would be set to "stats" or "results". However, although
that table would allow us to add arbitrary reports easily, I found the
way we had to handle the `has_many` relationship was a bit too complex.

app/controllers/dashboard/polls_controller.rb

@@ -15,8 +15,7 @@ class Dashboard::PollsController < Dashboard::BaseController
   def create
     authorize! :manage_polls, proposal
 
-    @poll = Poll.new(poll_params.merge(author: current_user, related: proposal,
-                                       stats_enabled: false))
+    @poll = Poll.new(poll_params.merge(author: current_user, related: proposal))
     if @poll.save
       redirect_to proposal_dashboard_polls_path(proposal), notice: t("flash.actions.create.poll")
     else
@@ -54,7 +53,7 @@ class Dashboard::PollsController < Dashboard::BaseController
     end
 
     def poll_attributes
-      [:name, :starts_at, :ends_at, :description, :results_enabled, :stats_enabled,
+      [:name, :starts_at, :ends_at, :description, :results_enabled,
        questions_attributes: question_attributes]
     end