Refactor force delete procedure

- Change `domains.force_delete_at` database column type to date,
rename to `force_delete_date`
- Save `force_delete_date` in application timezone
- Improve tests

Fixes #812, #253

db/migrate/20190322152123_change_domains_force_delete_at_to_date.rb

@@ -0,0 +1,5 @@
+class ChangeDomainsForceDeleteAtToDate < ActiveRecord::Migration
+  def change
+    change_column :domains, :force_delete_at, :date
+  end
+end

db/migrate/20190322152529_rename_domains_force_delete_at_to_force_delete_date.rb

@@ -0,0 +1,5 @@
+class RenameDomainsForceDeleteAtToForceDeleteDate < ActiveRecord::Migration
+  def change
+    rename_column :domains, :force_delete_at, :force_delete_date
+  end
+end