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.

config/locales/en/admin.yml

@@ -1049,8 +1049,6 @@ en:
       new:
         title: "New poll"
         show_results_and_stats: "Show results and stats"
-        show_results: "Show results"
-        show_stats: "Show stats"
         results_and_stats_reminder: "Marking these checkboxes the results and/or stats of this poll will be publicly available and every user will see them."
         submit_button: "Create poll"
       edit: