Run DB rake tasks on all tenants

Some tasks don't have to run on every tenant. The task to calculate the
TSV is only done for records which were present before we added the TSV
column, and that isn't going to happen in any tenants because we added
the TSV column before adding the tenants table. Similarly, the migration
needed for existing polls isn't necessary because there weren't any
tenants before we allowed to set the starting/ending time to polls.

We aren't adding any tests for these tasks because tests for rake tasks
are slow and tests creating tenants are also slow, making the
combination of the two even slower, particularly if we add tests for
every single task we're changing. We're adding tests for the
`.run_on_each` method instead.

lib/tasks/settings.rake

@@ -2,7 +2,7 @@ namespace :settings do
   desc "Add new settings"
   task add_new_settings: :environment do
     ApplicationLogger.new.info "Adding new settings"
-    Setting.add_new_settings
+    Tenant.run_on_each { Setting.add_new_settings }
   end
 
   desc "Rename existing settings"